I've noticed ever since I switched to Firefox 1.5 the fan on the Powerbook has been turning on more often. I pulled up Activity Monitor to see what was causing the CPU to heat up, and low and behold, its Firefox. Taking up about 60 to 70% CPU time. Not good.
Though I do enjoy being able to visit some Firefox/IE only Web 2.0 sites, I don't think its worth it. Especially when Safari is faster, more stable, and works with most of the sites that I use on a daily basis.