Is this with all websites/pages, some, a few?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Yes, all webpages on Firefox, all messages in Thunderbird. Mouse worked in both, I could cut and paste text into both. Everything normal on all other apps and browsers on the machine. Safe mode did not change it, neither did re-installing either app. However, quick Google search and it came up immediately, although it was referenced as a problem with Windows 10. Press Windows key and left shift button and all is back to normal. Windows problem, Mozilla problem, or both? Thanks for your effort!

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.