Review for Signature Switch by Petia Yvolk
Rated 4 out of 5 stars
There's no reason to delay upgrading TB to the last version. You can find a TB7-compatible version on the developer's site.
A very useful add-on. It would be still greater if it could auto-detect the language used in the e-mail and select the right signature accordingly.
To create your own collections, you must have a Mozilla Add-ons account.