Try uninstalling and reinstalling Firefox.
If that doesn't work, try setting Firefox preferences to default.
If Firefox won't start, you can try deleting the profile folder in
C:\Users\
your-username\AppData\Local\Mozilla\Firefox\Profiles\3t85k562.default
The 3t85k562.default folder would be the profile, and it will have a different number before the .default part. That number different with every installation.
Firefox should create a new profile automatically when you restart it after deleting the 3t85k562.default folder.
But if it doesn't, you can always uninstall and reinstall and that will create a new (uncorrupted) default profile.
You might want to try to export your bookmarks before you uninstall or delete the profile folder (assuming Firefox starts).
https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer