Sorry, this is an issue with older versions of Microsoft Office, they fail to follow Windows' lead regarding your default browser after you uninstall the old one.

When you uninstalled Firefox, most likely that removed the 32-bit program folder at

C:\Program Files (x86)\Mozilla Firefox

And the new installer installed Firefox under the 64-bit program folder at

C:\Program Files\Mozilla Firefox

Unfortunately, Office seems unable to "let go" of an old default browser when that program has been removed.

There is a registry hack you can use in this article: https://www.slipstick.com/problems/this-operation-has-been-cancelled-due-to-restrictions/

Or perhaps you can fool Office a different way -- I've never tried this before, but if you want to experiment:

(A) Exit out of all MS Office applications

(B) Copy the Mozilla Firefox folder from

C:\Program Files

to

C:\Program Files (x86)

(you can right-click > Copy, then right-click > Paste)

(C) Set the default browser to Internet Explorer

(D) Start Outlook and try a link to confirm that it now sends links to IE

(E) Have Firefox make itself the default browser again

(F) Test a link in Outlook to confirm that you're all done

(G) Remove the duplicate Firefox folder from

C:\Program Files (x86)

I used the "Edit Registry, Part 2" section of the article on hacking the registry and it seems to have worked fine. Thanks for your help, especially on Christmas Eve. Have a great holiday!