The recent upgrade to thunderbird-3.0.3 rendered the virtual identity extension unusable due to a version conflict.
The virtual identity extension allows you to edit the sender field when composing a new mail. Furthermore it warns you when trying to send a mail to a recipient you used another sender address for, before. This is especially useful if you use multiple accounts in thunderbird since you surely don´t want to write business mails from your private mail account.
The extension itself works with thunderbird-3.0.3, but thunderbird-3.0.3 is not in the range of compatible versions specified in the Addon package. It is possible to override this. To do so install MR Tech Toolkit via Tools->Addons and restart Thunderbird. Now, when installing an Addon you get an additional checkbox to deactivate the maxVersion test. Also in the context menu of the extension list there is now a menu entry “override compatibility” which allows you to activate installed Addons that fail the version test. Afterwards you can activate or install virtual identity again.
Edit: V0.6.4 is out and the procedure described above is not necessary any more for virtual identity.
The URLs for virtual identity above do not seem to work anymore. XPIs for the extension can be found here: ftp.uni-erlangen.de