In Firefox, open Responsive Design Mode (RDM), enter the width and height you need in the controls above the page, then click the camera button to save a viewport screenshot. For a size you will use again, save it as a custom device. Use the Web Console’s :screenshot helper when you need controls such as a device pixel ratio, delay, filename, or full-page capture.
Set a custom viewport size for a one-off screenshot
- Open Responsive Design Mode. In Firefox, choose Responsive Design Mode from the Browser Tools submenu, or use the Responsive Design Mode button in the Developer Tools toolbox. The keyboard shortcut is Ctrl + Shift + M on Windows and Linux, and Cmd + Opt + M on macOS. Mozilla’s keyboard shortcut list documents the shortcuts.
- Enter the dimensions. Edit the width and height values above the viewport. The initial content area is documented as 320 × 480 pixels. To adjust a value precisely, focus it and press the Up or Down arrow key; hold Shift while pressing an arrow key to change it by ten pixels. You can also drag the viewport’s bottom-right corner. See Mozilla’s Responsive Design Mode documentation.
- Capture the viewport. Click the camera button. Firefox saves the screenshot to its default download location. If “Screenshot to clipboard” is enabled in Developer Tools Settings, the capture is copied to the clipboard as well.
Manually entering dimensions is the quickest way to make a single screenshot at a particular viewport size. It changes the simulated content viewport; it does not, by itself, mean you have configured every device property.
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Save a reusable custom device
When you regularly need the same viewport and emulation settings, use Add Custom Device in Responsive Design Mode and save a named profile. Mozilla’s documentation says, “You can create and save custom devices in Responsive Design Mode by clicking the Add Custom Device button.” The profile can include size, device pixel ratio (DPR), user-agent string, and touch-screen behavior.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsChoose one-off size editing when you only need a single capture. Save a custom device when you want a named profile you can select again, particularly if the screenshot must reproduce more than width and height.
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Viewport size is not the same as the full device simulation
Width and height set the simulated content viewport. A device profile can also affect DPR, touch simulation, and the user-agent header or property. Mozilla documents that selecting a device sets screen size, DPR, touch simulation, and user-agent behavior. If your screenshot must reflect those properties, check the selected profile rather than relying on dimensions alone. See Responsive Design Mode and Mozilla’s Web Console Helpers documentation.
Use the Web Console for more screenshot controls
The RDM camera button is convenient for a straightforward viewport capture. Firefox’s Web Console :screenshot helper offers additional output controls, including --dpr, --delay, --filename, --clipboard, --file, and --fullpage. Mozilla’s screenshot documentation also describes --selector for capturing an element. Consult Taking screenshots for the helper’s syntax and options.
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Use the console helper when you need to control how the screenshot is saved or capture a full page or selected element; use the camera button when a viewport image is all you need.
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