Rated 5 out of 5 stars

I have Signature working in FF3, and this is how I did it:
http://planetzarg.blogspot.com/2008/06/install-signature-extension-for-firefox.html

Alternatively, have you tried inoculate128's method in the comments below?

This review is for a previous version of the add-on (0.4.0.5.20070419).