Mozilla Labs keeps up the good work launching tons of interesting add-ons for Firefox.  The latest, Geode is no exception.  The idea here is simple, to always know where you are.  Using the example they give, this extension makes a lot of sense:
You’ve arrived in a new city, a new continent, a new coffee shop. You don’t really know where you are, and are looking for a good place to eat. You pull out your laptop, fire up Firefox, [...]