Ryan Paul blogged at ars technica about a relatively simple way in which people who'd like to help improve Firefox 3 can get involved in testing the software.
"To participate in a test day, download the latest Firefox 3 nightly build from the Mozilla FTP server, join the #testday IRC channel on irc.mozilla.org, and use Mozilla's Litmus quality assurance tool to run test cases."
Mozilla's quality assurance community site has announced a series of test days for specific sets of features. September 7: Extension manager and content handling September 14: Bookmarks and history September 21: OS integration and site-specific preferences There's more information available on how to get involved with Mozilla here.
Becoming a part of the Firefox quality assurance process [ars technica]