In previous versions Firefox stored this information in the worst possible format - Mork. Format extremely complicated, barely machine readable that gave the users no possibility to parse the data (for a lot of laugh check out the specification on the Wikipedia page). This was finally replaced in the new Firefox by SQLite - self contained, small footprint database. Having database store your browsing data has one drawback, you have to optimize the database from time to time to get rid of old, deleted data, re-index the columns etc. Otherwise you keep all the junk data mixed in with your real data. In SQLite is the statement to do this called VACUUM;. So here's how to vacuum your Firefox. - Сова, удочеряющая носок