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)
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
Now re-zip the CONTENTS of the silvermelxt folder, (the contents are:
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).
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.