For those having this problem, here's the EASIEST solution. Yeah - tried that, with all possible combinations, no help. Probably because I cannot find anything named just "firefox" for that command to invoke.
You will if you look in /usr/bin/. /usr/bin/firefox is a symbolic link to /usr/lib/firefox-3.5.5/firefox, (or whichever version you're running) which is the actual executable. (All that should change is the version number...)
So far I've tried editing the config file with every possible combination of short and full paths, etc., from the config editor in Thunderbird, and by editing ~home/.thunderbird-3.0/<cryptic stuff>.default/prefs.js by hand using Kate. No cigar as yet. I'm missing something really stupid, because I've seen this several times before (and it was always something really obscure, but really simple). Since my last post I've even tried calling Konqueror to see if Firefox was the problem and, no, that didn't work either.
What I don't get is why Firefox and Thunderbird, two products from the same group of developers, won't play nice with each other just by default. This should be a no-brainer. I wonder if this is a file system standards thingy?
I'm intrigued by the suggestion to create an /etc/thunderbird.conf. It's all I haven't tried yet. Okay, I guess that's next.
(Sooner or later, enough of us monkeys at enough keyboards are gonna figure this out by dint of brute force if nothing else...