Update 3.1 Rated 5 out of 5 stars
@David. Sure you can think of viruses. But I'm not going to upload it to Mozilla because I've validated it up to version 3,5
How to do it yourself:
Via 7Zip open the XPI. Select the file install.rdf and edit these lines:
Close your text editor and 7Zip will ask to update the package. Now you can install the XPI in thunderbird.
I've added on the version a 'b' to see the difference with the original one.