"If you are a web developer or have a site that you would like to test during our release candidate cycle, we invite you to join this list. We will announce and communicate when builds are ready to test by using this list, so you will have a chance during the release candidate cycle to identify any possible issues *before* we push bits live. We may also periodically send out alerts when we need focused testing in an area", reads a fragment of a message from Mozilla.
And when I said that you will have the chance to be among the first to try out release candidates and developer builds of new iterations of the open source browser, I wasn't exaggerating in the least. In fact, Mozilla has promised that testers will be getting new beta builds of Firefox 3.0 and 2.0.0.x even before they will receive the email notifications. All you have to do is go over at the betatesters info page and sign up.