Based off of Autotrans by Glenn Powell. The original can be found at https://addons.mozilla.org/en-US/firefox/addon/autotrans/
Please note, I'm unlikely to be updating this addon in the future, except for getting it to work with new versions of Firefox.
This adds a context menu item which can be clicked to send the selected torrent link to the IP of your Transmission RPC through AJAX.
The context menu item can be setup to only display if the current link is a torrent file (with .torrent extension and correct mime-type).
You can set up username/password authentication for the RPC calls, as well as set the values for AJAX timeout and retry count.
There is also an optional menu item and status bar icon to direct you to your Transmission Web UI.