Try installing the auto tab discard addon (It is a recommended addon so it gets checked by Firefox). I’ve noticed certain tabs may slow FF down if they aren’t unloaded in the background.

I had FF slow to a crawl and when I checked it was using 7gb’s of ram. Turns out a site I had turned off uBO in had opened up a sub frame that was blasting ads. So it is interesting to try to see what is going on with about:processes.