Firefox’s --headless --screenshot command takes a screenshot without opening a GUI, but Mozilla’s command-line reference does not document it as a full-page capture method. To capture content beyond the viewport using a documented Firefox feature, open the Web Console and run :screenshot --fullpage. For scripted captures, Mozilla’s Marionette screenshot API supports full-frame capture, subject to a preference that can limit it to the viewport.
What “headless” means—and what it does not
In Firefox, --headless means the browser runs without a graphical user interface. It does not, by itself, mean the saved image contains the entire document. Mozilla documents startup screenshot options including --screenshot and --window-size, but its command-line reference does not describe a full-document option for that startup screenshot workflow.
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That distinction matters: a screenshot can be taken without a GUI and still show only the screenshot area. Increasing the window size changes the dimensions used for the startup screenshot; it is not documented as a way to capture the whole page regardless of its length.
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Capture the full page with Firefox’s Web Console
Mozilla documents the DevTools Web Console helper :screenshot --fullpage for capturing beyond the current window bounds. This is a command entered in Firefox’s Web Console, not a flag to append to a headless startup command.
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- Open the page you want to capture in Firefox.
- Open the Web Console. Use Firefox DevTools’ Web Console panel.
- Run the full-page command:
:screenshot --fullpage - Check the saved image to confirm that the page’s content below the viewport is included.
The helper also documents --filename to specify a file name, --delay to wait before capture, and --dpr to control device-pixel-ratio scaling. These are Web Console helper options; they should not be treated as Firefox startup screenshot flags. For example, to request a delayed full-page capture with a filename, use the helper’s documented options in the Web Console, consulting the helper syntax for your Firefox version.
Use Firefox’s screenshot button instead
If you prefer a GUI control over typing a console command, Firefox DevTools can show a button labeled “Take a screenshot of the entire page.” Enable it in DevTools Settings under Available Toolbox Buttons, then use the button to save the full page. Mozilla documents that this workflow saves the image to the browser’s Downloads directory.
What the headless command can do
For a regular startup screenshot with a defined capture size, Mozilla documents this command-line pattern:
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firefox --headless --screenshot /path/to/page.png --window-size 1365,900 https://example.comThis illustrates the documented headless, screenshot, and window-size options. It is not an authoritative full-page command: the cited command-line documentation does not establish that it captures document content beyond the screenshot dimensions. Check firefox --help on the machine where you will run it, because available options can vary by Firefox build and platform.
Automate full-frame screenshots with Marionette
For browser automation, Mozilla’s Marionette screenshot API describes a full argument that defaults to true for capturing the complete frame. The scope can be affected by the Firefox preference remote.screenshot.use_readback: when enabled, the readback path sees only pixels currently composited on screen, so remote screenshot captures are limited to the viewport. Mozilla documents this preference as false by default.
This caveat applies to the remote screenshot path; it should not be generalized to the Web Console helper or every Firefox screenshot workflow. If an automated capture unexpectedly contains only the visible area, check whether this preference has been enabled in the environment running Firefox.
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Choose the method that matches the job
| Method | Captures beyond the viewport? | Where it runs | Best fit |
|---|---|---|---|
Startup --headless --screenshot |
Not established by the cited command-line reference | Firefox command line | Simple no-GUI screenshots with configurable screenshot dimensions |
Web Console :screenshot --fullpage |
Yes; the helper documents full-page capture | Firefox DevTools Web Console | A direct, documented full-page capture |
| DevTools screenshot button | Yes; the button is for the entire page | Firefox DevTools interface | Interactive captures without entering a console command |
| Marionette screenshot API | Supports complete-frame capture; the remote readback preference can limit scope | Firefox remote automation | Programmatic browser automation |
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The headless screenshot shows only the visible page area
That result does not establish a failure: the startup screenshot reference does not document full-document capture. Use the Web Console’s :screenshot --fullpage helper or a documented full-frame automation route instead.
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The Web Console command is not recognized
Check that you entered the command in Firefox’s Web Console, not in a system terminal or as part of the startup command. The helper belongs to the DevTools console workflow.
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Check whether remote.screenshot.use_readback is enabled for that Firefox remote screenshot environment. Mozilla documents that enabling it limits the readback capture to currently composited viewport pixels.
The output dimensions differ from what you expected
For the startup screenshot path, --window-size specifies screenshot dimensions, not a documented full-document switch. For the Web Console helper, review the helper’s filename, delay, and DPR options and the output produced by your installed Firefox version.
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The command behaves differently on another machine
Verify the installed Firefox build and platform-specific options with firefox --help. The command-line options available can depend on the build and platform.
Frequently Asked Questions
Does --headless automatically make a Firefox screenshot full-page?
No. It runs Firefox without a GUI; full-document capture is a separate capability.
Can the Web Console full-page helper be used as a startup flag?
No. :screenshot --fullpage is a DevTools Web Console helper.
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