Some people have apparently reported that Firefox auto-updated to 1.5.0.1, but there was no official announcement of the update. Turns out that the update was a release candidate, & the only people who got the update were those who installed FF 1.5b1 or b2, then used the auto-update feature to update to 1.5 final. If you were one of those people, you un-officially became a beta-tester.

"Since you installed a beta and never re-installed Firefox from the full installer your update “channel” is beta - which means you are willing to run beta versions of Firefox. You can confirm this by opening the file {firefox install dir}\defaults\prefs\channel-prefs.js in a text editor. If line 2 looks like this:"