Firefox uses too much memory or CPU resources - How to fix and Firefox hangs or is not responding - How to fix may shed some light. What were the results of the troubleshooting steps?

If that does not help,

Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences<Enter> in the address bar

Select Advanced > General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

See also:

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

Are you using "Clear history when Firefox closes" to clear cookies or does one of your extensions do this?

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

• http://kb.mozillazine.org/Cookies
• https://support.mozilla.org/kb/Deleting+cookies