I upgraded to Firefox 3 yesterday and it's certainly a lot faster on my MacBook and Mac Mini than Firefox 2, as well as being cosmetically a lot more pleasing. One perplexing problem, though, was the appearance of a gray bar at the top of the window, like this:
When a browsing action was taken, a spinning pinwheel (the "Activity Indicator", apparently) would appear in that gray bar. I couldn't figure out how to eliminate this gray bar for a while but then hit upon a "Duh!" solution that I thought worth documenting here, and on the Firefox support forums. The bar is there because, for some inexplicable reason, the upgrade process moved the Activity Indicator into a separate space from the rest of the Navigation Toolbar. To fix it, right-click on the toolbar, select Customize... and then drag the Activity Indicator into the regular Navigation Toolbar. Click OK and the empty toolbar should disappear.