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

Sorry, this occurs when

(A) Firefox is your default browser

(B) Without changing your default browser, Firefox is removed from its original location

Some versions of Office get stuck and don't just go along with the changes at the Windows level.

One possible change along these lines would be if Firefox was uninstalled from "C:\Program Files (x86)\Mozilla Firefox" -- the normal folder for 32-bit Firefox on 64-bit Windows -- and then installed in "C:\Program Files\Mozilla Firefox" -- the normal folder for 64-bit Firefox.

There is not a nice, neat, clean fix for this, unfortunately. Please see the following article for hacky workarounds: https://www.slipstick.com/problems/this-operation-has-been-cancelled-due-to-restrictions/

Thanks for your patience and help. A computer guru friend solved the problem. It was neither Firefox nor Outlook. It was related to Windows 10.