Not sure it's related but a lot of Google's sites are terrible on Firefox and Edge. IIRC they are using polyfills for some heavy stuff (if you assume no malice, that's probably the cause of many problems).

https://www.reddit.com/r/firefox https://support.mozilla.org/

Try using safe-mode https://support.mozilla.org/en-US/kb/troubleshoot-firefox-is... or maybe creating a new profile. The beta versions, just like Chromium or Chrome, occasionally have some weird bugs. At least for me most of them are always hardware acceleration related.