Why does newer Firefox versions hang when opening multiple windows on startup, but older versions didn't?

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).