The final releases in the Firefox & Thunderbird 1.5.0.X builds series will be released sometime after Firefox 2.0 which as of today is still October 24th. Mozilla though is encouraging folks to move over the 2.0 build series since like the 1.0.X build series, there will no longer be any more updates on the 1.5.0.X build series after this release.
There are many great features (click links for related articles) in both Firefox 2.0 as well as Thunderbird 2.0 that they are worth upgrading to. The Firefox 2.0 download is a little larger at 7MB compared to 5MB for 1.0.8 and 1.5.0.7, but you get so much more included with this newer version, the biggest being inline spellcheck and session management.
