Actually, just the extension manager menu item has been turned on, and a non-working dialog for updating extensions (hence the "pre-preview" - it is not a mistake). This can be seen in today's (2004-04-21) nightly builds - I was alerted to this by this CVS comment Ben Goodger gave on a checkin of his:

turn on extension manager menu item (testing only!)

Just a note before the screenshots: none of this really works yet! So don't go around asking why the extension manager doesn't work if you do get the 2004-04-21 nightly build.

An "Extensions" menu item has been added to the "Tools" menu:

Screenshot of Extension Manager menu item

Clicking on that menu item opens the extensions manager, which supposedly would list the installed extensions and allows you to uninstall or update your extensions. Clicking on "Update" brings up the update dialog, which of course doesn't work at this point.

Screenshot of Extension Manager update dialog

Also available in the options (Tools -> Options) is a new "Software Update" section in the "Advanced" section.

Screenshot of Extension Manager options

Firefox will check for updates to your installed extensions via a web service (Ben Goodger has a weblog entry on this). Good stuff that. Checking for updated extensions is a difficult task to keep up with when you've anything over 3 extensions installed (I have 9). And there's going to be checking for new Firefox versions as well, judging from the options. Don't suppose it will be too useful for those of us who are in tune with the developments (nightly build users included, of course), but I suppose something like this is so quite standard in most applications now, and probably trivial to implement. Good for "normal" users I guess. Nightly build followers and contributing developers will probably know by instinct that a milestone release has been released. If not news will probably spread like wildfire from sources like MozillaZine and, of course, weblogs. Not going to use that option, I guess.

Update: Ben has just posted several screenshots of the Extension Manager.