Ben Goodger says in this thread:

Extension-Manager enabled builds should be available today on ftp.m.o []. Please start using these and test the new EM, file bugs!

"Today" being 24 May of course. The Extension Manager is partially functional now in branch builds (which you can get from the Mozilla Firefox nightly FTP site - just look for dates ending with "-0.9"). Get one of these builds, either 2004-05-24-0.9 build or the very fresh 2004-05-25-10-0.9 build (of course, if there are later builds by the time you read this, you should get those instead).

Screenshot of Extension Manager installing Download Statusbar extension

Only extensions that have been updated to work with the new Extensions API will be reflected in the Extension Manager (other extensions should install fine, but they won't be listed in the Extension Manager - they appear in Tools -> Options -> Themes as before). I tried out Content Holder and ContextMenu Extensions just to see how they work.

Screenshot of Extension Manager just after installing compatible extensions

Turns out they install just fine in a download manager-like kind of way. And after a restart of Firefox, they show up just fine.

Screenshot of Extension Manager with compatible extensions installed

The "Checking for Updates" for extensions doesn't work correctly yet - it shows a "New Updates Available" notification whether or not there actually are any new updates.

Screenshot of new extension updates notification

Lots of functionality is not there of course, but it helps to know that things are moving along just fine. Go Team Firefox!