Never had that problem even at FF’s worst. Try deleting your profile and starting over.
For me, currently, my setup is the best browser I've ever used.
Some forks have clear advantages, are well maintained and trustworthy.
There are numerous reasons to maintain a user.js file also, not just performance reasons.
Indeed, report issues to Mozilla directly, but that isn't inconsistent with using a fork and maintaining a user.js
That is of course not the case.