It works!
Date: 09/13/05
(Mozilla) Keywords: browser
roachfiend's How to update old extensions is cool, but as I've discovered: With 1.5B1 it's also possible to do it the old fashioned way.
Save the xpi to your desktop (or anywhere convenient) and just drag it into
restart the browser and voila! In some cases I had to restart the computer. Yeah, but that's just my machine being it's usually buggy self.
EDIT: If the install.rdf's maxversion reads 1.0+, replacing that with something specific like 1.9 makes the roachfiend tweak work.
Source: http://www.livejournal.com/community/mozilla/318962.html