Dont just make unsubstantiated claims because you cant configure it.
Theres nothing making it not work on manjaro when it would work on Arch, etc.
For my part it seems to work fine.
What packages do you have installed that you believe are required?
What firefox settings have you configured?
These firefox options are probably the ones in
gfx.webrender.all > true gfx.webrender.enabled > true layers.acceleration.force-enabled > true # media.ffmpeg.dmabuf-textures.enabled > true media.ffmpeg.vaapi-drm-display.enabled > true media.ffmpeg.vaapi.enabled > true # media.ffvpx.enabled > false
(the ones commented out are probably not required but may be nice)