Thunderbird Not Opening Firefox Upon HTML Link Click

I have been sorting out issues with Firefox and more recently Thunderbird. I think I might have Firefox moving along as advertised, but I have a new mole in my tech themed Whack-a-Mole game. The newest mole head is explained in the thread title.

I did peruse the settings in Thunderbird, but I could not find anything. Is this a common issue? If so, is there an easy remedy? If not, any suggestions.

And MarkTrent, you have been the man as of late. I bet you respond!

(For the record, a previous "Thank You" has been sent).

Thanks for any help. Oh yeah, all are the newest versions, whatever they happen to be.