Until now I had to right-click the link to torrent file, choose "Copy link location", open a new tab, type the URL to Transmission's Web UI, click "Open" button, paste the torrent link and click "Upload" button. Now it's simply a right-click on the link and select "Torrent it!". Bam!
Add-on configuration is pretty straight forward. Enter the IP/host name of the server the Transmission daemon is running on (you can even enter multiple daemons). If your daemon requires authentication, you can enter username and password. If you want, you can tweak values for AJAX timeout and retry count. There is also an optional menu item and status bar icon that takes you to Transmission web interface. The Autotrans is also smart enough to only show the context menu when link is a torrent/magnet link.