From MozillaZine Knowledge Base
Firefox 3 introduced a number of improvements to the Location Bar. Among them were changes to how sites are ordered in the dropdown menu. Firefox takes into account how frequently and how recently sites in the browser history were visited, combining them into a statistic called “frecency.” The frecency of sites is calculated when the browser is idle. The way frecency is computed can be tweaked by a number of preferences.
The means by which a user arrived at a given URL (the “transition type”) is taken into account when calculating frecency. Typing in a URL, for example, is given a higher “bonus” than visiting a bookmark or clicking a link. These preferences—of the form places.frecency.(visit type)VisitBonus—determine the various bonus percentages given to the defined transition types.
Possible values and their effects
The percent bonus given to the various visit transition types.
By default, the following preferences and values exist:
First checked in
Has an effect in