Try creating a new Profile and if still have the issue : https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles If that fixes the issue running the new profile then : https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues If fails : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/ Please let us know if this solved your issue or if need further assistance.

It is possible that some of the status keys have the wrong state and Firefox sees them as still pressed. Try to press all the status keys (Shift, Ctrl, Alt) a few times to see if that helps to recover from this. If it doesn't help then close and restart Firefox or reboot the computer.