I understand that Firefox is stuck on the "Clear History" screen, I am happy to help.

• Firefox hangs or is not responding - How to fix

You should be able to close windows like about:preferences and the Library window via "Command+W".

You can delete xulstore.json in the Firefox profile folder to recover from this issue.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox