Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Thanks for the response, I have a lot of tabs open (way too many!). Will this delete them, or will I get them back?

Thanks!

A new profile will remove them, but best is first to test a new profile as clean as possible to verify that it works and be cautious with copying files from the old profile.

Your open tabs are stored in the sessionstore.js file in the profile folder and if this file is very large then this can cause problems.

• http://kb.mozillazine.org/Session_Restore

Went on Safe Mode, it looks like it's one of the extensions. Going to have to do some experimenting...