Try this: https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding.

What is the file size of the sessionstore.js file in the Firefox profile folder? You can use this button to go to the currently used Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data. *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Multiple_profile_files_created Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).