That can be a problem with the compatibility.ini file in the Firefox profile folder. Try to rename (or delete) the compatibility.ini file in the Firefox profile folder to see if that helps. 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) *http://kb.mozillazine.org/Profile_folder_-_Firefox You can also check for problems with preferences. Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. *http://kb.mozillazine.org/Preferences_not_saved *http://kb.mozillazine.org/Resetting_preferences

Deleting all the .js files in the profile cleared all the session start problems that I was experiencing following the update to 34.0.5