Rated 5 out of 5 stars
Thanks Lemon Juice!
I am using SeaMonkey 2.49.1 because I found that Thunderbird (on 3 Windows 7 machines) frequently hangs with a blank message window when trying to view a new message. (Latest Courier IMAPD server.) Clicking some other message and then the desired one sometimes works . . . I didn't feel like updating to a more recent SeaMonkey, since this one works.
Not being able to select another browser than SeaMonkey is an odd omission, which your add-one fixes, for me, perfectly.
(SeaMonkey now does the same hanging behaviour, so perhaps this is a server problem and unrelated to the client . . .)