The last update to Firefox 2 was 2.0.0.20. Now FF 2 is no longer in development. However, if you look throught the mozill.org ftp site, it looks like they are still working on it.
I would assume the 1.8 engine is still in development. Its used in other applications like Camino and Seamonkey that may have not updated to the 1.9 engine yet.
-= Hmm… I guess they still work on it but with less priority because I heard that Mozilla is working on for Firefox 3 & Firefox 4 Alpha [Codename: Minefield]…