Tied with Safari for my favorite browser. Both are extremely fast and blow Internet Explorer out of the water. Internet Explorer is the only application that regularly crashes on OS X; not so with Safari or Mozilla which are much more stable. Mozilla seems to cache content much better than Safari leading to better speed gains for certain sites. The default appearance is fairly ugly in Mozilla---the modern theme is a bit better, but the nicest theme is Pinstripe which gives Mozilla a clean Aqua appearance. A new version of Pinstripe which will be compatible with Mozilla 1.5b will be released shortly at:
http://www.kmgerich.com/pinstripe/pinstripe.html