Is this the same problem?

https://support.mozilla.org/en-US/questions/1012222#answer-607655

I am not the only one with this problem, which I didn't have before updating Mozilla. So what are Firefox going to do about it?

Have you tried restarting Thunderbird with add-ons disabled from the help menu to see if it is an add-on that is causing your issue?

Thanks for your help, but I've just tried what you suggested without any success.

There are two separate problems here: if you can't copy a URL, that is a problem with Firefox or whatever program you're copying from. After you copy the URL, paste it directly into the body of a TB message with Ctrl-V. If it works, then the copy operation is working.

Next, select Insert/Link (Ctrl-L), type in the Link Text, then click in the Link Location box (not right-click), then Ctrl-V.