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To capture a webpage at a specific viewport size in Firefox, open Responsive Design Mode (RDM), set the width and height above the page, then click RDM’s camera button. The capture shows the page rendered in that viewport; use Firefox’s separate screenshot tool when you need the visible area, an entire page, or a custom crop.
Capture a page at a chosen viewport size
- Open the webpage in Firefox.
- Open Responsive Design Mode from the Browser Tools submenu, the Developer Tools toolbar, or press Ctrl+Shift+M on Windows/Linux or Cmd+Opt+M on macOS. Mozilla documents these entry points in its Responsive Design Mode guide.
- Edit the width and height values displayed above the viewport. Set both values explicitly; RDM’s documented starting size is 320 × 480 pixels.
- Wait for the page to render at the selected dimensions. If you changed the user agent or touch simulation and the page checks those values when it loads, reload it so its code can respond to the changes.
- Click the camera button at the right of the viewport dimensions. Firefox saves the screenshot to its default download location, unless Developer Tools Settings has Screenshot to clipboard enabled, in which case it copies the image to the system clipboard.
For fine adjustments, edit the numbers directly, use the Up and Down keys (one pixel per press, or ten pixels with Shift), scroll the mouse wheel, or drag the viewport’s bottom-right corner. Mozilla documents these controls in the RDM guide.
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Choose the screenshot mode that matches your goal
| What you need | Firefox method | What it captures |
|---|---|---|
| A page rendered at specified viewport dimensions | Responsive Design Mode, then its camera button | The displayed viewport at the selected width and height |
| The part of the page currently on screen | Right-click an empty part of the page and choose Take Screenshot, or press Ctrl+Shift+S (macOS: Command+Shift+S), then choose Save visible | The currently visible portion |
| The entire document, including content beyond the viewport | Open Firefox’s screenshot tool and choose Save full page | A full-page image, not a viewport-sized image |
| Only a particular region | In the screenshot tool, click and drag to select a region; resize or reposition it if needed | The selected crop, which you can download or copy |
The screenshot tool’s current choices and keyboard controls are described in Mozilla’s Take screenshots in Firefox guide. Use RDM when the requested result depends on the browser viewport’s dimensions; a full-page screenshot has a different scope.
Set device properties when size alone is not enough
Viewport width and height do not define every aspect of a mobile layout. RDM can also set device pixel ratio (DPR), touch simulation, and user agent. Selecting a listed device applies its screen size, DPR, touch simulation, and the user-agent header for that device’s default browser. You can also create a custom device with a name, dimensions, DPR, user-agent string, and touchscreen setting. See Mozilla’s Responsive Design Mode documentation for the available controls.
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- For a simple responsive breakpoint check, set the target width and height directly.
- For a device-oriented comparison, select a matching device profile or configure the relevant DPR, touch, and user-agent settings as well as the viewport dimensions.
- Do not treat emulation as a guarantee of a perfect physical-device match: these settings simulate selected characteristics, not every feature of a real phone or tablet.
RDM also offers network throttling, but Mozilla describes its results as approximate. It is useful for a rough emulation, not an exact performance measurement.
Troubleshoot common capture problems
- The image has the wrong dimensions: Check both RDM width and height before capturing. The separate screenshot tool’s visible or full-page options do not set a custom viewport.
- The page still looks like the previous device or user agent: Reload after changing a property that the site reads on page load. Also check that the intended RDM reload setting is enabled when you need the page to rerun code after an emulation change.
- You cannot find the saved image: Check Firefox’s default download location. If Screenshot to clipboard is enabled in Developer Tools Settings, look on the clipboard instead of expecting a downloaded file.
- You need the whole page, not just the viewport: Use Save full page in Firefox’s screenshot tool. RDM’s camera captures the displayed viewport.
- Keyboard crosshair movement does not work: Mozilla notes that moving the screenshot crosshair does not work on Wayland. Use the mouse to select the region instead.
- The capture does not reflect a real phone exactly: Confirm the intended viewport, DPR, touch simulation, and user agent. RDM emulates selected properties; it does not establish a full physical-device match.
Or skip the browser setup
If you need a repeatable capture in code rather than a manual Firefox screenshot, ScreenshotNeo takes a screenshot or PDF from one API request. For example, this cURL request captures Stripe’s webpage:
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