UPDATE: If you're using Firefox 76 or above, this should be fixed. The issue was caused by libdrm-2.4.101. Bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1623885
- open https://webglsamples.org/aquarium/aquarium.html and note the framerate
- open about:config, find security.sandbox.content.read_path_whitelist and set it to /sys/
- restart firefox, open aquarium again
What's happening here is firefox (actually mesa, because of firefox security sandbox) having a problem determining the proper DRI driver to use, with amdgpu kernel module at least it tries to use amdgpu_dri and fails, since that's part of AMDGPU-PRO. Its own sandboxing is causing it, given access to /sys/ it's using radeonsi_dri as it should be.
This makes huge difference even with default basic compositing, many things are using webgl, for example google maps, the performance difference is obvious. You can also enable gfx.webrender.all and gfx.webrender.compositor for few fps more and less cpu usage (on my A8-7600 I'm getting 11fps by default, 40fps and 40% cpu usage with basic compositing but whitelist modified, and 45fps/30% cpu usage with webrender+compositing enabled - doesn't come close to 60fps with 10x the amount of fish in brave/chromium, but it's progress and google maps changes from stuttery mess to perfectly usable).