Rated 4 out of 5 stars
To make it work in TB 3, I had to proceed as follows (the code change suggested in ZeBoxx's review didn't work for me):
Step 1: change em:maxVersion in install.rdf to 3.*
Step 2: edit chrome\copyattachmenttoclipboard.jar::chrome\content\copyattachmenttoclipboard.js as follows:
Replace "attachment.messageUri" with "attachment.uri" (one occurrence).
I have uploaded the modified xpi file here:http://www.megaupload.com/?d=YQH5NQEK
Disclaimer: I have no previous experience with Mozilla plugins, so this may or may not work for you.