I tried that out for a while when it was first announced, but it didn't seem to work as well as, say, Android's Intents. And as you say, appears to have gone stale.
I suppose this is the scenario that Web Intents (http://webintents.org/) are intended to deal with.
...and there actually seems to be something going on with regards to sharing in Firefox still. According to [1] it's targeted at Firefox 17, though I haven't noticed anything yet (I'm on Firefox Aurora 17 now). It seems like it has a much broader scope than just sharing.
Sharing something easily is one thing, but they seem to be planning to provide a newsfeed/wall with this. That'd be a big distraction that I hope they keep away...
For people spending their days on Facebook it might be an improvement. It does sound like a huge addon though and I agree that it'd probably be best to make most of that non-default.
The problem is, how keenly will the various services let something like Firefox take their newsfeed? This is exactly what Twitter for example is trying to prohibit with their ToS. Just sharing is a lot more durable.
See https://wiki.mozilla.org/Labs/F1 and http://f1.mozillamessaging.com/