How does one install extensions in Firefox and Thunderbird?

[Ken Heard <ken@heard.name>]

	I recently downloaded two extensions, one each for Firefox 1.5 and
Thunderbird 1.5.  The Firefox extension was installed in
file:///usr/lib/firefox/extensions/preferential.xpi, and the Thunderbird
 one in file:///var/lib/thunderbird/extensions/configdate-0.3.7-tb.xpi
linked from file:///usr/lib/thunderbird/extensions.

	When I start each application the "Software Installation" window pops
up and asks me whether I want to install the extension in question.
However, when I answer yes by hitting the "Install Now" button, and then
go to the Tools>Extensions window, the extension is not listed.  Each
time I start those applications the same thing happens.

	I am using Sarge with both Firefox 1.5 and Thunderbird 1.5 installed
from the Debian backports site.  Can anyone explain what my problem is?
--
				Ken Heard
				Research Associate
				Museum Studies Program
				University of Toronto, Canada