For the vast majority of Thunderbird users out there, this extension is useless.
However, those of us in the minority find it a Godsend.
OK, "Godsend" is a bit of a stretch but it's a damn nice little extension. :).
It comes in very handy (especially if your mail is on a host that has multiple domains - e.g. most webhosting companies).
The "mismatched" notice was getting old. But thanks to this extension, it's toast. :D