Ben Goodger (Firefox engineering lead) has announced is targeting a 12th June or 14th June release date for Firefox 0.9. The Extension Manager will be fully enabled in this build, as will the new update mechanism for themes (and extensions, of course).

When it's released, be prepared to see go down again (as was the case with the 0.8 release of Firefox) thanks to the Slashdot effect. Hoard your extensions and themes in advance (well, at least those extensions which are not yet 0.9-compatible). Remember that BitTorrent exists if the servers go down. You have been forewarned.