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Firefox is well known for its large library of add-ons which can be used to add new features or modify the behavior of existing features. Firefox's "Add-ons Manager" is used to manage installed add-ons or find new ones.
Once set, these affect the user's current profile, and may be synchronized across all devices via Firefox Sync. Please note that only a subset of the about:config entries are synchronized by this method, and the exact subset may be found by searching for services.sync.prefs in about:config. Additional preferences and third party preferences may be synchronized by creating new boolean entries prepending the value with services.sync.prefs.sync. To synchronize the whitelist for the extension NoScript:
Firefox also allows configuration for a profile via a user.js file: user.js kept also in the profile folder. A user.js configuration supersedes a prefs.js. The user.js configuration is only parsed at start-up of a profile. Hence, you can test changes via about:config and modify user.js at runtime accordingly. For a useful starting point, see e.g custom user.js which is targeted at privacy/security conscious users.
One drawback of the above approach is that it is not applied system-wide. Furthermore, this is not useful as a "pre-configuration", since the profile directory is created after first launch of the browser. You can, however, let firefox create a new profile and, after closing it again, copy the contents of an already created profile folder into it.
Sometimes, it may be desired to lock certain settings, a feature useful in widespread deployments of customized Firefox. In order to create a system-wide configuration, follow the steps outlined in Customizing Firefox Using AutoConfig:
Firefox uses PulseAudio for audio playback and capture. If PulseAudio is not installed, Firefox uses ALSA instead. Note that by default, Firefox blocks all media with sound from playing automatically [4].
Widevine is a digital rights management tool that Netflix, Amazon Prime Video, and others use to protect their video content. It can be enabled in Settings > General > Digital Rights Management (DRM) Content. If you visit a Widevine-enabled page when this setting is disabled, Firefox will display a prompt below the address bar asking for permission to install DRM. Approve this and then wait for the "Downloading" bar to disappear; now, you are able to watch videos from Widevine protected sites.
VA-API usage can be verified by checking Firefox's VA-API logs. Run Firefox with the MOZ_LOG="FFmpegVideo:5" environment variable and check in the log output that VA-API is enabled and used (search for the "VA-API" string) when playing a video for example. Pay attention to these logs as they might indicate that only one of the two possible compositors described before (WebRender or OpenGL) works with VA-API on your particular setup.
To enable spell checking for a specific language, right click on any text field and check the Check Spelling box. To select a language for spell checking, you have to right click again and select your language from the Languages sub-menu.
Starting with version 64, Firefox can optionally use XDG Desktop Portals to handle various desktop features, such as opening a file picker, or handling MIME types. Using Desktop Portals allows you to, for example, customize which program is invoked to display a dialog when you select files to upload on a webpage or when picking a download location using Save as.... See XDG Desktop Portal#List of backends and interfaces for a list of available backend options.
TTS must be setup for the Listen icon to appear in the Reader view. Firefox uses Speech dispatcher which requires a speech synthesis engine. The currently recommended speech synthesis engine is Festival.
The Listen icon (a headphones icon) will only appear if you have performed all the configuration above, Speech Dispatcher is working, and you have started Firefox after you started the Festival server (you cannot start Firefox then Festival).
The voices in the package festival-us provide better quality audio than those in festival-english but they do not work in Firefox. They do not appear in the list of available voices in Firefox and when you open Reader view you will see error messages like this in the terminal output from the Festival server:
Firefox should respect your GTK theme settings and your OS-wide dark appearance settings (as in the Appearance section of GNOME's settings or KDE system settings). If the latter does not work, make sure to have a suitable xdg-desktop-portal package installed.
Starting with Firefox 68, you can make all the Firefox interfaces and even other websites respect dark themes, irrespective of the system GTK theme and Firefox theme. To do this, set ui.systemUsesDarkTheme to 1 in about:config [13].
As of Firefox 100, further control of the dark theme of web pages that opt-in (using the CSS media query prefers-color-scheme) and Firefox's own in-content pages is possible with layout.css.prefers-color-scheme.content-override. Setting this to 3 will follow the browser theme, setting this to 2 will follow the system wide dark-mode preference (ui.systemUsesDarkTheme as above, which defaults to 0 if the user has not changed the dark-mode preference or if a system does not support a system-wide dark-mode preference), while 1 and 0 will always force light-mode and dark-mode respectively. This setting can also be accessed through the user settings of Firefox under General > Language and Appearance > Website appearance.
You can Take a Screenshot by either using the screenshots button that can be added to the toolbar from the customize screen in the Hamburger menu at More tools > Customize toolbar, by pressing Ctrl+Shift+s or by right-clicking on the webpage. See [15] for more information.
As an alternative, you can use the screenshot button in the developer tools, which can be added through the developer tools Settings menu, under the Available Toolbox Buttons section. The settings for the developer tools are accessible through the three horizontal dots located at the top right of the developer tools pane.
Starting with version 121, Firefox defaults to Wayland instead of XWayland and does not require any configuration. Older versions of Firefox support opting into Wayland mode via an environment variable.
To verify that it worked, look for Window Protocol in about:support. It should say wayland. The presence of x11 means you are running Firefox under Xorg display server, while xwayland means your system is running Wayland but executing Firefox as legacy X11 application.
To be able to apply different configurations to Firefox windows, change the WM_CLASS string by using Firefox's --class option. Under Wayland, Firefox uses the --name option instead. You can then reference separate Firefox windows in your window manager by using the strings you set.
Kinetic scrolling feels loose on Wayland due to _bug.cgi?id=1568722, and can be turned off by going to about:config and turning off apz.gtk.kinetic_scroll.enabled. This will make it harder to scroll to the beginning and end of long pages, however.
Firefox supports kiosk mode that shows pages in full screen without browser chrome, context menus, and other features useful for typical desktop browsing. These can be seen on ATMs or information panels where users are not expected to interact with the rest of the system.
Starting with Firefox version 89, the compact mode density option was removed from the Customize panel [16], but you can still use compact density. To do this, set browser.uidensity to 1 in about:config.
Firefox includes a search provider for GNOME Shell which exposes Firefox bookmarks and history to GNOME Shell search while Firefox is running. However, this provider is disabled by default; to enable it go to about:config and set browser.gnome-search-provider.enabled to true.
This mode can also be enabled by pressing on the hamburger menu while Firefox is open, clicking Help, selecting Troubleshoot Mode and confirming this on the modal dialog that appears. Please note this will require a browser restart.
If you have ruled out other causes, it may be worth trying a new Firefox profile for testing purposes to see if this will resolve your issue. More information on how to create a new profile and switch between profiles can be found on the Firefox support page.
Refreshing your profile will retain all browsing and download history, bookmarks, web form auto-fill data, cookies, personal dictionary and passwords, and will transfer them to a brand new profile without extensions, themes, extension data and preferences, among other data. A backup of your old profile will also be retained.
To refresh your profile, navigate to about:support, press Refresh Firefox and confirm this on the modal dialog that appears. about:support can also be accessed by pressing the Hamburger menu, selecting Help and then clicking More troubleshooting information.
If you are having issues with hardware video acceleration in Firefox, e.g. in case of freezes or graphical corruption, start Firefox in Troubleshoot Mode for testing purposes to confirm that this is the issue. If this step resolves the issue, merely set media.ffmpeg.vaapi.enabled to false in about:config to disable hardware video acceleration, and restart Firefox.
By default, extensions will not affect pages designated by extensions.webextensions.restrictedDomains. If this is not desired, this field can be cleared (special pages such as about:* will not be affected). Then create and set privacy.resistFingerprinting.block_mozAddonManager to true.
If Firefox takes much longer to start up than other browsers, it may be due to lacking configuration of the localhost in /etc/hosts. See Network configuration#Local network hostname resolution on how to set it up.
Firefox has a setting which determines how many replacements it will allow from Fontconfig. To allow it to use all your replacement rules, change gfx.font_rendering.fontconfig.max_generic_substitutions to 127 (the highest possible value).
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