Could you try starting Firefox in "Safe Mode" (Firefox Safe Mode, not Windows Safe Mode)?

With all Firefox windows closed, hold the Shift key when starting Firefox. When you get the Safe Mode dialog, don't check any boxes, just click Continue in Safe Mode.

Can you start up in Safe Mode?

More info: Diagnose Firefox issues using Troubleshoot Mode

solved

Hi lisak, hopefully it will work on your next startup, when you are no longer in Safe Mode. But if not, you can use the Safe Mode dialog to disable add-ons or try other steps to bypass the problem.

It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).

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

You can try to reset Firefox and create a new profile if you keep having problems.

• https://support.mozilla.org/kb/reset-firefox-easily-fix-most-problems