Firefox 3 is looking like it's going to be a great improvement over version 2, however as a developer I'm always interested standards compliance testing like the ACID tests because the better browsers do, the easier it gets for people creating web apps.
Just to keep track, here are how the latest browsers stack up as of June 5 2008 on the Acid 3 Test:
Firefox 3 RC2: 71
Safari 3.1.1: 75
Opera 9.5 Beta 2: 78
IE 8 Beta 1: 17
Firefox 18.104.22.168: 54
IE 7 Vista SP1: 5
By the way, various browses have private builds tucked away that score better in the dark of night, but these scores represent the very latest publicly advertised betas and released products. If it's not good enough to highlight on their web site, it's not ready to brag about its score.