Firefox 8 is now available on the nightly channel. If you are already running Nightly (Firefox 7.0a1), you may be able to update by going to Help > About Nightly. Otherwise you may be directed to the Nightly Download Page on Mozilla Central.
Note: At this time, the merge for Aurora from Firefox 6 to Firefox 7 as well as Beta from Firefox 5 to Firefox 6 have not yet been completed. We will post when these merges have been completed.