Short answer: Firefox’s documented headless command-line screenshot captures a chosen window size, but Mozilla does not document a full-page flag for that command. For an explicitly full-page capture, Mozilla documents the Web Console helper :screenshot /absolute/path/page.png --fullpage. The documentation reviewed does not establish how to run that helper in a headless-only session.
What Firefox headless mode can capture from the command line
Firefox supports command-line screenshots with --screenshot. The switch itself implies headless operation, and --window-size sets the screenshot dimensions. For example:
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firefox --headless --screenshot /absolute/path/page.png --window-size=1440,1000 https://example.com/This saves a screenshot at the chosen dimensions; it is not a documented capture of the entire document. A tall window may show more vertical content, but Mozilla’s command-line reference does not say that it captures everything below the viewport. See Mozilla’s Command Line Parameters and the older Firefox headless-mode article.
The command-line reference says the screenshot is saved to the requested path, or to the working directory if no path is supplied. Use an absolute path when a script needs a predictable output location. Check firefox --help on the machine that will run the command: available options can vary by build and platform.
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Use the Web Console for Firefox’s documented full-page capture
Mozilla documents the DevTools Web Console helper :screenshot with the --fullpage option. It captures portions of the webpage outside the current window bounds:
:screenshot /absolute/path/page.png --fullpageEnter this in Firefox’s Web Console; it is a Console helper, not a documented headless CLI flag. Mozilla’s documentation does not establish a headless-only way to invoke this helper. Consult Web Console Helpers, Taking screenshots and DevTools Tips for the documented Console workflow.
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Other documented helper options
--delaywaits a specified number of seconds before capturing; fractional values are supported.--dprsets the device pixel ratio.--filenamespecifies a PNG filename.--selectorcaptures one CSS-selected element and its descendants.--fullpageincludes content outside the current window bounds.
If you capture again using an existing filename, Firefox overwrites the previous image. Choose a unique filename or move the earlier capture if you need to keep it.
Choose the method that matches the requirement
| Method | Where it runs | What the cited documentation establishes | Best fit |
|---|---|---|---|
firefox --headless --screenshot ... |
Command line | A screenshot at specified dimensions; no documented full-document flag | Shell automation when a viewport-sized capture is sufficient |
:screenshot ... --fullpage |
Firefox DevTools Web Console | Capture beyond the current window bounds, with options such as delay, DPR and selector | A documented whole-page capture when using the Console workflow |
These are not interchangeable full-page methods: only the Web Console helper is explicitly documented with --fullpage. If your requirement is both command-line automation and a whole-document image, the cited Firefox documentation does not provide a headless-only procedure for that combination.
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Make sure the page is ready before capturing
Dynamic pages may add content after initial navigation, including images loaded as the page scrolls. Mozilla’s cited documentation describes the helper’s delay option, but does not guarantee that a delay will load every lazy image or late-rendered element. If completeness matters, confirm the page has finished rendering and that the content you need is present before capture.
In the Console workflow, use --delay when a known short wait is appropriate. It is a timing control, not a guarantee that a site’s asynchronous work has completed. The headless CLI reference cited here does not document an equivalent full-page readiness mechanism.
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Troubleshooting
- The CLI image stops at the visible area:
--window-sizesets dimensions; it is not a documented full-page option. Use the Web Console helper with--fullpagefor the documented whole-page workflow. - Firefox rejects an option: run
firefox --helpand check the installed build’s available options. Command-line options vary by build and platform. - The output is missing or in an unexpected folder: specify an absolute output path. Without a requested path, the CLI saves to the working directory.
- A later capture replaced an earlier image: the helper overwrites an existing filename. Use a different filename for each capture you need to retain.
- The screenshot misses late or lazy content: verify the page is ready before capture; do not assume a delay alone guarantees that all dynamic content has loaded.
- A headless run appears to hang: a Mozilla Support thread describes one user’s Firefox 125-era Linux hang and a moderator’s suggestion to try a dedicated profile. This is anecdotal troubleshooting context, not evidence that Firefox generally hangs or that a dedicated profile is a universal fix. See the Mozilla Support thread.
Or skip the browser setup
If you need a screenshot without setting up Firefox capture steps, ScreenshotNeo offers a screenshot API and MCP server. One GET request returns an image or PDF. Example using cURL:
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallcurl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo API documentation for request options. ScreenshotNeo removes cookie banners, newsletter popups and chat widgets before capture; bot checks, blank pages and failed loads are never billed. Its MCP server lets AI agents take screenshots. The free plan includes 1,000 screenshots per month with no card, and paid plans start at $5 for 3,000.
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