Here's what I don't understand ...
You're willing to use FF5 (It's 13 versions behind) ... but you won't use FF3.6 which works perfectly without having to use the IE Tab.
I just don't get it.
I'm sorry to hear FF5 stopped working FBB. I'm still using FF3.6.28 and don't have a single problem. I did switch for a while and use the IE Tab, but there was something about it that didn't work right that I didn't discover until I used it for a few days. Can't remember what it was now, but I ended up switching back.
Every text formatting function works, all my icons stay visible when I zoom in and out, I can reply to people properly. The Search function on the forum doesn't work, but that's something Cake did, nothing to do with browsers.
Maybe you can give it a try just for kicks and giggles and see if it works for you.
Here's a
link to the Windows download page for FF3.6.28. I'm assuming it's the En-GB one you want, but I wasn't sure so I didn't link directly to the file.
Hope this helps in some small way.
Good luck,
Bub.