Rated 5 out of 5 stars

How to get it to work on TB 31:

Download the extension silvermel-1.5.7-fx+tb.xpi

Unzip this to into a folder silvermel-1.5.7-fx+tb with inside 3 files: install.rdf, silvermel.jar, silvermelxt.xip.
open install.rdf and change the Thunderbird Max version. (it is set to 28.*. I've set it to 38*, current version is 31, stuff still looks good with this addon enabled)

<!-- Thunderbird GUID -->
<em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
<em:minVersion>18.0</em:minVersion>
<em:maxVersion>38.*</em:maxVersion>
</Description>
</em:targetApplication>
<!-- Front End MetaData -->


Unzip silvermelxt.xpi into a folder called silvermelxt. Delete silvermelxt.xpi (you will create a new one).

open install.rdf insize the silvermelxt folder, and change the Thunderbird max version there too
<!-- Thunderbird GUID -->
<em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
<em:minVersion>18.0</em:minVersion>
<em:maxVersion>38.*</em:maxVersion>
</Description>
</em:targetApplication>
<!-- Front End MetaData -->

Now re-zip the CONTENTS of the silvermelxt folder, (the contents are:
3 folders
chrome,
defaults,
modules,
chrome.manifest,
install.rdf)

Call the file silvermelxt.zip. Rename it to silvermelxt.xpi.
Move this file into the parent folder silvermel-1.5.7-fx+tb, (there already is an install.rdf there, as well as a silvermel.jar, you already deleted the original xpi file, if you didn't delete it you now have to overwrite it.).

Now open the install.rdf that's inside the silvermel-1.5.6-fx+tb folder, and change that version to something higher as well.

You now have 3 files inside a folder called silvermel-1.5.7-fx+tb:
install.rdf (with changed max version).
silvermel.jar (unchanged)
silvermelxt.xpi (unzipped - changed install.rdf - rezipped - renamed to xpi).

Zip these 3 files into silvermel-1.5.7-fx+tb.zip, and rename that to silvermel-1.5.7-fx+tb.xpi. This is our new XPI., your new addon install file.
Save this new xpi file to a folder where you can find it again. (You can delete all the other files )
Install this new xpi in Thunderbird, and you have a working Silvermel addon.

This review is for a previous version of the add-on (1.5.7.1-signed).