lhns--steam-frame-nix/docs/firefox.md

Firefox

firefox.enable: a launcher for the Flathub Firefox Flatpak (org.mozilla.firefox, stable). It shadows the Flatpak's own entry (same ID), so default-browser associations keep working. Options: options.md#firefox.

Options

• vrFullscreenFix (on): gamescope never shows fullscreen windows, so Firefox looks frozen. full-screen-api.ignore-widgets makes fullscreen fill just the window. Not applied in the desktop profile. Remove when gamescope shows fullscreen X11 windows in VR.
• disableAv1 (off): media.av1.enabled = false. The Frame's decoder driver (iris) has no AV1, only H.264, HEVC and VP9, so YouTube and co. send VP9/H.264, decoded in hardware, instead of software AV1. Remove when a SteamOS kernel adds AV1 to iris.
• prefs: further about:config values for every profile; they can also override the fixes above.
• desktopProfile ("desktop"): the sessions can't see each other's Firefox, so a second instance stops at the locked profile; in the nested desktop the launcher uses this separate profile (a normal Firefox profile with its own browser data, created on first use). null: the default profile in both sessions.

Changes take effect at the next start of Firefox.

How it works

prefs and the fixes are default values (pref()), not user values: about:config can still change them per profile, and nothing is written to prefs.js, so removing one leaves nothing behind. Firefox reads default prefs from defaults/pref/*.js in its system config dir, which in the Flatpak is /app/etc/firefox, the mount point of the org.mozilla.firefox.systemconfig extension. home-manager provides that extension as a link from ~/.local/share/flatpak/extension/org.mozilla.firefox.systemconfig/aarch64/stable to a store directory; Flatpak mounts it itself, so the sandbox doesn't get /nix. (A systemconfig extension of your own would conflict with it.)

The desktop profile undoes the fullscreen fix only while its Firefox runs: the launcher links the profile's user.js to /app/etc/firefox/steam-frame-nix-desktop-user.js (a sandbox path) right before starting Firefox, waits for it, and once it has exited and the profile is no longer in use removes the link and the value Firefox stored from it in prefs.js. A second launch that just hands a URL to the running Firefox leaves both in place. A user.js of your own is never touched (the fix then stays on in that profile). After a crash, the next launch or steam-frame-nix-cleanup (on switch) removes them.

Older versions linked or copied a user.js into every profile; those and the values they left in prefs.js are removed by steam-frame-nix-cleanup on switch, for each profile not in use at that moment.