Does anyone know?

You can check the Firefox Task Manager (about:performance) and about:memory pages to see if there are memory problems.

• https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources

I checked, there aren’t any memory problems.

Going to give this a shot! Have you tried to clean out the Program Cache and the Browsing Cache? You can clear your Browsing Cache in Preferences-Browser Privacy. Also try clearing the program Cache you can get one of these from the Mac App Store ( I Prefer the one from Pocket Bits not a bad price). Also if you have not got it get OynX from Titanium Software it is totally free and above board just make sure you get the version that is for the version of your MacOs. Also if all else fails refresh Firefox back to defaults!

Clearing both caches didn’t work, nor did refreshing Firefox back to the default.

It almost sounds like you might have a extension that is giving a problem! If you have not done so remove all your extensions and see if it still does it if not install one extension at a time and see if you can find the problem! Also you might want to try completely removing Firefox from your system and try starting over again!