I just downloaded Firefox 1.5 RC2 for Linux and built the RPM for Fedora Core 4 using Thomas Chung's spec file from http://www.fedoranews.org. I just modified it to use the RC2 source instead of RC1. So far, Firefox 1.5 RC2 is running well on my PC. It's stable, no noticeable performance issues and not noticeable performance increases either.
I haven't yet found any websites with rendering problems either so overall, stable, smooth and clean.