Another Way to Make It Work Rated 5 out of 5 stars
As an alternative to riffian's method for making Signature work in FF3, try installing the Nightly Tester Tools extension. It has many functions, but you need only one: the feature which allows you to override the compatibility of individual extensions with a right-click. Don't use the "Override all compatibility" button, because it may get you in trouble with some other extensions. Either way you do it, though, Signature works just as well under FF3 as under FF2.This review is for a previous version of the add-on (0.4.0.5.20070419).