Rated 5 out of 5 stars

Works great. A bit labour intensive to configure, but it works great.

Concept is this:
-Thunderbird 3.x can still only read from a single signature file
-This add on writes new signatures into that file, at a frequency you specify
-You will need to populate another file, called a "fortune cookie" with the list of signatures you would like TB to use.
-Add on reads fortune cookie, grabs new signature, writes to the original file, TB reads from that file, and presto!, you have a fresh signature that will not match the last one.

I had 60+ signatures I was using in Apple Mail. By carefully aranging window placement, I was able to drag and drop from Apple Mail into the fortune cookie file. Having the delimiter (an "%" symbol on a line between each signature) in place before hand made it much easier to accurately drop the signature into the fortune cookie. It was still tedious but much easier than doing a copy and paste for 60+ times.

This review is for a previous version of the add-on (1.4).