To expand on this, there are two settings you can put in user.js / prefs.js (desktop) or via about:config (mobile), documented on the Mozilla Wiki:

user_pref("media.autoplay.default", 5);
user_pref("media.autoplay.blocking_policy", 2);

Two bonus settings if you want to get rid of the “do you want to enable DRM?” pop-in bar when hitting one of those sites:

user_pref("media.gmp-widevinecdm.enabled", false);
user_pref("media.gmp-widevinecdm.visible", false);

hth

The Arch wiki may have some ideas for you - tl;dr is that GDM uses a global dconf db over in /etc/ and this might be the root of your problem (these configs might not get cleaned up with a --purge?) I’m a LightDM user so best I can do to help: https://wiki.archlinux.org/title/GDM#dconf_configuration

Quick update for anyone still reading this thread:

@fdroidorg@floss.social As with any other app, we flagged Fennec and Mull with KnownVuln until the app is updated. Contributors fixed the issues that delayed versions 130 and later. Stand by for the build.

https://floss.social/@fdroidorg/113384089915217604

A bit of backstory on how we got here - in June 2024 Mozilla chose to (a) integrate the source tree of Firefox Mobile into their huge monorepo (“gecko-dev”), and (b) move the source off of Github onto their own git servers (“Mozilla Central”). You can read about it in the now-archived old repo:

• https://github.com/mozilla-mobile/firefox-android
• https://github.com/mozilla-mobile/firefox-android/wiki#upcoming-migration-to-mozilla-central

This was then compounded by a core Android build kit (“NDK”) choosing to remove parts of the toolchain which is/was used to build Firefox releases (ergo, forcing another change to build process):

• https://github.com/android/ndk/issues/2010

Together these have caused a bit of a kerfuffle in getting new releases compiled and released via the official F-Droid methodology. See the other comment about the Mull version in their private repo, they’re having to use a Mozilla pre-built clang (a compiler toolchain) now to make it work for the time being.

The link(s) to add their F-Droid repo if not running DivestOS: https://divestos.org/pages/our_apps.html#repos