Thanks or the suggestion. The Flatpak is indeed doing just fine but it's not an option for a number of reasons, such as the Flatpak version not working with certain addons (KeePassXC-Browser cannot connect to KeePassXC).

Well, looks like I have found the cause: the TearFree set to true appears to block HW compositing / acceleration in Firefox: https://wiki.archlinux.org/title/intel_graphics#Tearing I have now removed the config file and hardware accelerated video decoding now works just fine while running both Xfce and KDE Plasma.

Thanks for all the input!

VLC and mpv are working just fine with hardware acceleration, confirmed it with intel_gpu_top.

Still nothing on Firefox. Launched it with MOZ_X11_EGL=1, media.ffmpeg.vaapi.enabled is set to true. Tested with YouTube, still nothing. 😑