Have you tried a clean install of firefox? A separate profile even. A lot of people switching to another browser and finding it much more performant/stable is just them starting with a default browser with minimal addons.
Sometimes a clean install isn't even needed. I moved my ~/.mozilla directory across all my main PC installs (and different flavors such as Palemoon and Waterfox) for years and all I had to do was cleaning the cache and every now and then the years long history which can really slow down the browser at each startup (note: system disk on SSD, home on mechanical HD, so YMMV).
