lets start with a little trial and error.

if you open notepad and copy and paste some text from it to Thunderbird does paste work? Does the paste from Firefox to notepad work?

Note: Notepad only places basic text on the clipboard. This will suggest if the problem is pasting in general or the format of the data placed on the clipboard.

Yes, that works. Copying text from a web page displayed on Firefox to paste into an email doesn't work. Copying a URL to paste as a link doesn't work. I just tried copying text and a URL from Microsoft Edge and pasting into Thunderbird. Both worked fine, so I'm thinking the problem is not Thunderbird, but maybe Firefox.

Just before copying a URL from Edge, the copy/paste to/from Firefox and Thunderbird failed. After copying a URL from Edge and pasting it as a link in Thunderbird, I am now able to do the same from Firefox.

That is interesting weirdness..... Let us know if that changes.

Thanks for responding, Matt. So far, so good. It was broken for about 3 days through multiple reboots and shut downs. Somehow copying from a different browser reset something.