Firefox dropdown menu- how do things get put there?

The dropdown menu on firefox- the one that shows when you click on the down arrow to the right of the address bar- shows several sites that make sense would be there, google, yahoo, facebook, etc. But how do they get there? for instance, also on the list is a link to a specific Straight Dope message* thread* that I viewed and posted to a few times months ago. Why was that thread chosen, and not others since? Why a specific thread, and not simply Straightdope.com? (most of the other sites listed are just the domain, ie, espn.com)

Just curious about firefox’s methodology.

FF3+'s address bar, now called the “AwesomeBar”, combines frequency and recency into “frecency”.

Sounds silly, but read more about it if you’re interested: Frecency