According to Mozilla and other sources, Firefox 2 and Gecko 1.8 will soon be left behind some time in mid-December. The end result: no future security or stability updates. This will affect Thunderbird 2, SeaMonkey 1.1, Camino 1.5, and any other projects based on Gecko 1.8. So, if you haven't already upgraded, there's no time like the present.
I personally like Firefox 2 better than Firefox 3. To me it is just faster. Thankfully I save all of the installers.