Have you installed add-ons?

Might be a bug, so we can troubleshoot to try to figure it out.

If you visit about:processes, is there any website or extension taking up a lot of system resources?

In addition to that, you can try launching firefox in safe/troubleshooting mode. 3 bars in the top right > Help > Troubleshooting mode

This restarts firefox with all addons, extensions, and themes disabled.

For further troubleshooting, also try:

Go to about:profiles, and create and launch a new one. This is a more complete reset, since it also resets all settings, about:config options, and similar.

After creating a new profile, you can play around with rebuilding your current firefox config, adding extensions and changing settings.

Nope

Maybe something you have changed a long time ago in about:config that does not work out with a recent update ? I don't know if it's possible

Or add-ons as previously mentioned

Or DNS over HTTPS configured in Firefox that does not work anymore for any reason

Firefox is good for me on a lot of sites, but it doesn’t seem to like Lemmy as much as I do. So I use Safari for that (I’m on a Mac). And then I just keep using Safari because I really don’t hate it.

On the PC, it's almost unusable. Android, no problems.

I switched to Vivaldi when Jellyfin stopped working for whatever reason. Still ok on Android though.

No problems whatsoever for me