This is a port of blassey's simple tab-sharing front-end UX from a Firefox Mobile patch attached to bug 742832 to an Firefox Desktop XPI. It's a restartless XPI based on edile'es example restartless stuff.
The intent is to make it possible to experiment with various user experiences for tab-sharing.
https://github.com/dmose/tabshare/blob/master/TabShare.xpi?raw=true is the XPI; if you save it to disk and install it on a recent Firefox desktop nightly, the "share tab" in the permissions dropdown will offer the active tab as a source. This is actually a fairly bizarre UX; but it's a first step. It's slightly easier to see the potential if one uses it while playing around with http://talkilla.mozillalabs.com/
Note that the backend currently always only shares the top box of the document, bug 952625 is now on file to make it possible to share scrolling viewports as well.