I'd start by disabling all add-ons and check if that helps. If not, there's something wrong with Firefox itself.

If disabling them fixed the issue, I'd continue with a binary search to see if there's a specific addon causing issues. I.e. enable only half the add-ons and re-check. Then continue enabling/disabling add-ons until you know which addon causes the issue.

That said, it could be the combined load, or a combination of add-ons, which is significantly harder to fix.