Maybe a broken add-on.
You may want to try a clean profile to check that possibility.
From command line (or a changed shortcut) run Firefox with Profilemanager:
"C:\wherever\Mozilla Firefox\firefox.exe" -ProfileManager
1. There you can create a new profile which won't have any add-ons = browser default configuration.
2. Test your issue.
3. Run Profilemanager again to delete the empty profile and switch back to your standard profile.
Hope it helps as this is a very annoying one, indeed.