Since announcing support for HW video initially on Wayland, Firefox has been off to the races on this [0]. Despite requiring lots of hacks to enable flags as you linked on the Arch Wiki, one of which now suggests disabling the sandbox for content processes and a major red flag, it somehow worked.
For one reason or another [1], Firefox chose to lock the video drivers to the old i965 drivers instead of choosing the newer iHD driver. A weird decision that still applies to current builds.
There have been incompatibility issues with video codecs. In my experience I can play h264 videos with hardware acceleration but not vp9 with the flatpak distribution. With vanilla package, both h264 and vp9 video is hw accelerated.
I could also have sworn that video acceleration support has gotten worse over time since the introduction.
I now get a weird artifact at the bottom half of the screen only when watching YouTube videos. [2]
Video hardware acceleration is not entirely doomed on Linux. Even if Chromium does not want to make any move toward this direction in this century. Gnome web or rather Epiphany has stellar video acceleration support by using Gstreamer plugins. So it is possible to get hw video working on a browser.
[0] https://mastransky.wordpress.com/2020/06/03/firefox-on-fedor...