"Close Firefox" Error Won't Open F'fox

Why do I get the "Close Firefox" error when opening a link in T'bird 3.0.3? That used to happen on earlier versions as well. WHY DOES IT DO THAT? Why in the world would T'bird care if F'fox is open or not?

 

It depends on which browser is your default, because Thunderbird will want to open a link in the default browser. Now, if Firefox is default, the link should open with it (the link should show up in an opened Firefox, or open Firefox, if closed.
But, on the other hand, TB3.x is whacked out, and I refused to use it. I sill have 3.0.3, so I will test it.
My default mail is TB2.0.0.24, and default browser is Firefox 3.6.2.

P.S: I tested with Firefox 3.5.10 open, with TB3.0.3, and had no problem.
It took me a while to do the second test, i.e. with Firefox closed. I tried two links, and nothing happens. Finally, a third one opened the default Firefox 3.6.2.
And, I dislike TB3.0.3 even more, being so sluggish. It takes forever to move mail to Trash, or another folder. At least, to Trash, it must mark the message Read, before deleting it. This is incomprehensible. But, I never got a message like yours, although I have four different Firefox versions.