Could be any number of things. Possibly a bad plug in or combination of plug-ins. Could be your new video card didn't jive with your installation of Firefox.
Run Firefox in safe mode and see if the performance improves. If it does, you likely have a plugin causing the issue. From there, disable all of them and reenable them one at a time to see if the problem comes back.
Disable and reenable hardware acceleration.
Disable ipv6 This is an issue I ran into earlier this year.
Clear cache and cookies
And last resort, Uninstall Firefox and all profiles and settings and reinstall a fresh copy. (after syncing your account, obviously)