I updated to Catalina earlier this year and currently running version 10.15.6 on my 1 year old iMac. A few months ago I noticed problems with Firefox freezing and had to Force Quit often, almost always. Also, after using Firefox, my Mac would not shut down. The spinning fan would just hang. Sometimes it would restart, but the only way to shut down the Mac was to hold the power button. I've tried a number of different suggested solutions, including removing the one extension I was using; AdBlock. I cleaned the cache. I reset Firefox to default settings. Unistalled then reinstalled Firefox. Also, unplugged my external hard drive that was using Time Machine. Nothing has worked. Finally, I stopped using Firefox altogether and started using Safari. It's been over a week now and absolutely no problems, so I can only conclude that Firefox is not compatible with Catalina, at least on my instance.