Great !
However I would have two remarks. The extension doesn't seem to understand the native firefox format used for the feeds, no big deal though but the experience would be smoother I think. A more annoying point is that there is no indication if the link has been visited or not (make it bold for example ?), it should be feasible using firefox history.
Ah, and adding buttons to mark all entries as read or unread would be a nice addition (I come from sage too, does it show ?).

Anyway, such a simple extension that does not try to implement its own history different from the firefox one is really good news, thank you !