Well that's a start.
We have reboot covered, fans are going, it's only on Internet and Firefox has blue circle loading issues, since that occurs on the tabs so it's looking like a strickly Firefox issue.
Have you rebooted and not start Firefox, then tried Safari, does the issue still occur?
Check under Apple Menu > Software Update for any updates to OS X and or Apple programs and apply.
If it goes away using Safari, try uninstalling all Firefox add-ons and Firefox, then use Safari and check here for updates for your browser plug-ins.
https://www.mozilla.org/en-US/plugincheck/
Next install Firefox again from the same link above, it's version 20 now. But don't install any add-ons.
If your problem goes away then I think you found the problem.
Also check your Applications/Utilites/Activity Monitor and click on CPU to see if any process is consuiming more processor cycles than it really should be (like 100-200%) that indicates a runaway process and mention it here.