Chances are you're still using the old Greasemonkey script or the version included in the Better Gmail 2 add-on. These versions have been discontinued, are no longer supported, and are not very stable in Firefox 3. Consider un-installing these versions (or unchecking the Gmail HTML Signatures box in Better Gmail 2, then installing the latest stand-alone Blank Canvas Gmail Signatures add-on.

v1.07 was a Greasemonkey script that worked fairly well in Firefox 2, but is EXTREMELY flaky in FF3 (signatures did not appear to save most of the time). Uninstall this script completely.

The folks over at Better Gmail 2 included my v1.07 as an option in their add-on. This is the exact same logic and is as bugged as v1.07 (because it's just the same script repackaged.) Uninstall Better Gmail 2 if you don't use any other features, or make sure you uncheck "HTML Signatures" in the compose tab.

v1.08 was released as a stand-alone Firefox 3 add-on. The logic was re-written from the ground up and should be much more stable now. Since then, there have been other updates, with every 1.## change representing a change in features, and every 1.##.## representing bug-fixes within a feature change release.

You should always be able to find the latest version here.