Basic restartless add-on built from Brad Lassey's example code in bug 742832.
Once installed, you have a "share tab" camera option when starting a WebRTC session. If you decide to share a tab, you can choose the specific tab you want from a list.
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.