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Firefox has the strongest documented privacy posture for AI-assisted form filling when keeping processing on your device is the priority. Its Smart Window can use open tabs you choose as sources, generate suggestions locally, and let you review them before filling. Mozilla says ordinary saved autofill values are inserted directly from device storage: “Nothing leaves your device or is sent to any AI model.”
Brave is the better fit for readers who put the greatest weight on no-retention language. Brave says its Leo service does not retain prompts, responses, context, or personal data on its servers, but its autonomous AI-browsing feature remains early testing and introduces prompt-injection and unintended-action risks. Chrome offers the broadest documented field coverage—including passports, driver’s licenses, vehicle information, license plates, and VINs—but Google documents sending form metadata and submitted data types to improve predictions.
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The short answer
| Browser | What is documented | Best for | Main qualification |
|---|---|---|---|
| Firefox | Saved autofill is filled from data on the device. Smart Window uses user-selected open tabs and provides a review step. | Local processing and source-by-source control | Smart Window is limited to supported forms, English locales in the United States and Canada, and a staged rollout. |
| Brave | Leo says it does not retain prompts, responses, context, or personal data on its servers; browsing history is not collected or retained under Brave’s privacy policy. | Short-retention claims and privacy-oriented defaults | Autonomous AI browsing is early testing and can be manipulated by indirect prompt injection. |
| Chrome | Google documents broad autofill coverage and confirmation before some sensitive fills, while also describing transmission of form structure, field names, domain hashes, and submitted data types. | Coverage and mature prediction services | Encrypted storage does not mean all prediction processing stays on the device. |
There is no current vendor-published, head-to-head score for private AI form filling across these browsers. The choice depends on four separate questions: where processing occurs, what metadata leaves the browser, how long a provider keeps it, and how much control you have before a value is inserted or submitted.
What “private AI form filling” actually means
Browser autofill and AI-assisted filling are related but not identical. Conventional autofill inserts values that you have already saved, such as a name, address, password, payment card, or telephone number. AI-assisted filling tries to infer what a field means, find a suitable value in another source, or operate a sequence of browser actions on your behalf.
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Privacy therefore has several dimensions:
- Processing location: whether a model and the source information run on your device or are sent to a provider.
- Metadata exposure: whether the browser transmits field labels, form structure, domain information, or the types of data you submit even when it does not send the completed values.
- Retention: whether prompts, responses, browsing context, or personal data are stored, cached, or associated with an account.
- User control: whether you choose the source material, inspect proposed values, and confirm sensitive actions.
- Coverage and maturity: which kinds of forms are supported, in which countries and languages, and whether the feature is a stable autofill function or an experimental agent.
A browser can perform well on one dimension and poorly on another. “Private” should not be treated as a single score.
Firefox: the clearest local-processing story
How Smart Window fills a form
Firefox’s Smart Window recognizes supported forms and can use open tabs that you explicitly select as sources. It suggests values for fields that ordinary Firefox autofill cannot complete. Before filling, you can review the proposed sources and deselect a tab you do not want used. That source-selection step is important: a résumé tab, for example, can be used without granting the feature access to every tab in the window.
The control is labeled Fill from sources. Mozilla documents it as appearing only on supported forms with at least five text fields that Smart Window can help fill. If the control is absent, the form may not meet those conditions, the feature may not be available in your locale, or the rollout may not have reached your account.
What Mozilla says stays on the device
Mozilla distinguishes Smart Window suggestions from ordinary saved autofill. Saved names, addresses, passwords, and payment information are inserted directly from data stored on the device. Mozilla’s support documentation states: “Nothing leaves your device or is sent to any AI model.” That is the strongest explicit local-processing statement among the three browsers for the saved-autofill path.
Mozilla’s broader privacy notice also says that some AI features use small language models downloaded to the device. For those features, page content, PDFs, images, and tab URLs remain on the device unless you explicitly consent otherwise. This matters when the source is a document containing employment history, identity data, or financial information.
Availability and practical limits
Smart Window is currently documented as available only in English locales in the United States and Canada. Availability can also depend on the form itself. A form with fewer than five helpful text fields may not expose Fill from sources, and unsupported controls or unusual field markup can prevent useful suggestions.
Local processing does not guarantee correct processing. Mozilla warns that generated suggestions can be wrong and instructs users to review them before submitting. A résumé may contain several dates, job titles, or addresses that look plausible in the wrong field; a local model can still make that mistake without sending data anywhere.
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Firefox’s privacy boundary
Firefox is the strongest choice when your threat model prioritizes keeping saved autofill data and selected source content on the device. The claim should not be expanded into “every Firefox AI feature is always local” or “every form is private by default.” Smart Window has documented locale and form limitations, and ordinary web pages can still send information to the site when you submit the completed form.
Chrome: the widest documented coverage, with off-device metadata
What Chrome can fill
Google’s November 3, 2025 product announcement describes expanded form understanding in Chrome. With permission, Chrome can save and fill passport numbers, driver’s-license numbers, vehicle information, license plates, VINs, and other fields. Google says the stored information is protected with encryption and that Chrome asks for confirmation before filling saved information on your behalf.
That breadth is useful for government, travel, and vehicle forms where a basic name-and-address profile is not enough. It also increases the consequences of a mistaken match, so the confirmation step should be treated as a required review rather than a nuisance.
What Google receives for predictions
Chrome’s help documentation says it sends information about form fields to Google to offer suggestions. The documented information includes the basic structure of the form, field names, and a hashed version of the site domain. When you submit a form, Chrome sends information about the structure and attributes of the form and the types of data submitted to improve future predictions and Password Manager.
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This is a meaningful privacy distinction. The documentation describes a Google-assisted prediction service, not a purely local model. A hashed domain is not the same as an unmodified URL, but it is still metadata leaving the browser. Likewise, sending data types and form structure can reveal that a page requested a passport number, vehicle identification number, or another sensitive category even if the full value is not included in the prediction request.
Encryption versus local processing
Chrome’s announcement says saved information is encrypted and stored only with permission. Encryption protects stored data from unauthorized access, but it does not change where prediction processing happens. A privacy-conscious user should evaluate those as separate protections: encryption addresses storage, while Google’s autofill documentation addresses transmission for prediction.
When Chrome is the rational choice
Chrome makes sense when completing many different kinds of forms is more important than keeping form metadata entirely on the device. Its mature prediction infrastructure and broader documented field set can reduce manual entry, particularly for travel and vehicle paperwork. The trade-off is explicit: Google’s documentation describes sending form information to Google, so Chrome is not the best fit for a strict local-only requirement.
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Brave: strong no-retention language, experimental agentic behavior
What Brave says about Leo
Brave’s privacy policy says it does not collect or retain a user’s browsing history. For Leo, Brave says it does not collect identifiers that can be linked to the user and does not store or retain prompts, responses, context, or personal data on its servers. Brave notes that it may cache large prompts to improve performance, with deletion within minutes.
Those statements address retention and account linkage rather than proving that every operation is local. They are especially relevant if your concern is a provider building a persistent profile from form-filling conversations or browsing context.
Why autonomous form filling changes the risk
Brave’s AI-browsing announcement describes the feature as early testing and warns that giving an AI control of browsing can expose personal data or cause unintended actions. The announcement identifies indirect prompt injection as a systemic challenge for the category of AI-powered browsers.
An indirect prompt injection is instructions embedded in a page, document, or message that try to redirect the agent. A job application could contain hidden or misleading text telling an agent to disclose information, change a destination, or submit before you have reviewed the answers. This risk exists even when a service promises not to retain prompts after the session.
How to use Brave cautiously
Keep autonomous actions behind a confirmation requirement. Let the agent draft or populate fields, but do not allow it to submit an employment, identity, health, financial, or government form without your inspection. Treat every page the agent visits as untrusted input, including pages you normally consider reputable.
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Brave is compelling for readers who value no-history and no-retention language, but its experimental agentic workflow should not be confused with a conventional, narrowly scoped autofill database.
What independent autofill testing tells us
A 2020 empirical study by researchers at the University of Illinois at Chicago found that Chrome autofilled 46.5% more sites than Firefox and 63.1% more forms overall in the tested scenarios. The study also reported that Firefox was the only browser in one tested hiding technique that filled concealed elements.
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These figures are historical observations about conventional autofill, not a current benchmark of Firefox Smart Window, Chrome’s 2025 enhancements, or Brave’s 2026 AI-browsing implementation. They are useful for understanding why coverage and hidden-field behavior matter, but they cannot establish a present-day privacy ranking.
Hidden fields are a security concern because a page can include inputs that are not visibly presented to you. A browser that fills more forms may be more convenient while also increasing the importance of checking which fields received values before submission. The study’s results reinforce the need for a visible review step, not a claim that one browser is currently safer in every situation.
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Résumé, cover-letter, and recruiting forms
Firefox is the clearest fit when you want to select a résumé or cover-letter tab as the source and keep that content on the device. Review every suggestion: job histories often contain repeated employers, overlapping dates, and multiple locations that can be assigned incorrectly.
Passports, driver’s licenses, and vehicle records
Chrome has the broadest documented coverage for these categories. Its confirmation prompt is valuable because a wrong passport number or VIN can be difficult to notice after submission. If your policy forbids sending form metadata to Google, the coverage advantage does not outweigh that requirement.
Health, financial, and government applications
Use the browser whose data path matches your organization’s rules, then perform a field-by-field review. Firefox offers the clearest documented local path for saved autofill; Chrome offers explicit confirmation for some sensitive fills; Brave’s agentic mode should remain supervised because pages can issue indirect instructions.
Forms with unusual or hidden fields
Expect inconsistent behavior. The 2020 study shows that browsers differ substantially in how aggressively they autofill and how they treat concealed elements. Before submitting, scroll through the complete form, inspect the page’s visible confirmation summary if one exists, and clear any value that does not belong.
A privacy-first workflow that works in any browser
- Classify the form. Decide whether it contains ordinary contact data, credentials, identity numbers, health information, financial details, or government records. The more sensitive the category, the less you should delegate.
- Limit the source. In Firefox, select only the tabs that Smart Window needs. In an agentic workflow, give the assistant the narrowest possible task and avoid leaving unrelated sensitive tabs open.
- Generate suggestions without submitting. Treat the first pass as a draft. Do not let convenience turn a fill action into a submission action.
- Check field meaning, not just spelling. Confirm that dates, document numbers, addresses, job titles, and vehicle identifiers belong to the exact field label shown.
- Inspect for hidden or unexpected fields. Look for values inserted into fields you did not intend to complete. Historical testing demonstrates that hidden-field behavior varies by browser.
- Confirm the destination. Verify the organization, domain, and recipient before sending. A correct value submitted to the wrong site is still a privacy failure.
- Submit manually. Keep the final click under your control for employment, identity, health, financial, and government forms.
- Clear temporary sources. Close or lock documents and tabs containing sensitive information when the task is finished, especially on shared devices.
Common misunderstandings and failure modes
“Encrypted” means “processed locally”
It does not. Chrome documents encryption for saved information and separately documents sending form metadata to Google for predictions. Evaluate storage protection and processing location independently.
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“No retention” means “no risk”
It does not. Brave’s no-retention language reduces the risk of persistent server-side storage, but an AI that can browse and act can still expose information during the session or follow a malicious page instruction.
“Local” means “accurate”
It does not. Firefox’s Smart Window can produce incorrect suggestions, which is why Mozilla instructs users to review them. Privacy protects the data path; it does not validate the answer.
Autofill and the website are the same privacy boundary
They are not. A browser may keep a value local until you submit, after which the website receives whatever the form sends. Browser privacy controls cannot make a recipient’s own data practices private.
Cross-browser portability and password managers
Saved passwords, addresses, and payment records are handled differently from AI-generated suggestions. If you move between browsers, a privacy-focused password manager can provide a more consistent encrypted vault and review process than relying on one browser’s profile. It also separates credential storage from experimental AI features.
Use a password manager for secrets and let AI-assisted filling handle only the fields you are prepared to verify. Never treat an AI suggestion as a substitute for a password vault, multifactor authentication, or a manual check of an identity number.
Final decision
- Pick Firefox when local handling and explicit source-tab selection are your deciding criteria, provided you are in the supported English-language US or Canadian rollout and your form exposes Fill from sources.
- Pick Brave when no-history and no-retention claims matter most, while keeping autonomous browsing supervised and treating every page as a possible prompt-injection source.
- Pick Chrome when you need the widest documented field coverage and accept that Google receives form metadata and submitted data types for prediction and product improvement.
For all three, review the completed form before submission. No privacy claim removes the need to verify the recipient, the field values, and the final action.
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