But no compositing will have other issues: Cairo Dock not work as expected.

Are you looking for a solution? Which is more important if you had to choose? Compositing or a FF which works without hogging the CPU?

Did you try what I suggested?

In my case the problem was resolved when I disabled the display compositing. Please at least try it to see if it fixes anything.

Believe me, I don't like disabling compositing at all, but it did resolve my problem.

If what I suggested solves your problem with FF, but you feel that you must have display compositing on, I don't know what to tell you short of trying other Desktop Environments and/or Window Managers.

You might also ask in some other subreddits and/or web forums, mailing lists.. So that's about all the help I can try to provide you for this. Best of luck to you and have a great weekend.