Firefox 1.5 Release Candidate 2 is available now! There are several fixes to automated update system, and I must say it works very smooth.
This afternoon I waited until the update feature of Firefox 1.5 RC1 initiate me to update to RC2. Now, the update is very small unlike the previous Firefox 1.x versions where you need to download a full 4.7+ Mb installer to upgrade. This particular upgrade from RC1 to RC2 only needs 390kb download. 
Links:
Firefox official website
MozillaZine