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
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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