Apparently, people are starting to discuss Firefox 1.5 versus IE7. As Asa writes, it's a bit stupid to compare an unreleased browser with one that has been released. There's no estimate when the final version of IE7 will be released (well, I haven't seen any dates) and when it is released, it'll only be available for Windows XP and Vista. Firefox 2.0 might be released by the time IE7 arrives so if you want to compare, compare IE7 with that instead.
Asa from the Mozilla headquarters is writes more about this.