Rated 4 out of 5 stars
Works great, and is the only REAL solution for BiDi (right-to-left) messages.
The only pitfall is the fact that it sometimes causes TB to revert the text to 'Variable width' instead of using the setting that was set under 'Options > Composition > HTML > Font'. It's a bit irritating that when you go to the end of a line (press 'End'), the font changes.
But all-in-all, an excellent work!
What you describe seems to be Thunderbird's behavior rather than ours - I don't touch the font that way. But please file a bug at www.mozdev.org/bugs/ and try to provide me with instructions on how to reproduce this bug.