I use Librewolf on desktop and Mull on mobile. I have a few extensions on both, which could definitely contribute to issues. When I have issues (usually government sites or financial stuff, sometimes DRM-related stuff for media) it’s easier to just use a Chromium-based browser with no extensions than try to troubleshoot specifically what’s causing the issues. I keep Falkon (desktop) and Vanadium (mobile) installed for this purpose.

I get the feeling a lot of issues people are having in Firefox might be due to extensions or settings, which gets “fixed” by using another browser (which happens to be Chromium-based because most browsers are) and they blame the issue on Firefox itself.