Open Tools > Options > Privacy. Change the drop down to "Use Custom Settings for History" and then you will find the culprit sitting quietly over there. There is a default setting inFirefox to remember history for 90 days which is really huge. This basically means that Firefox will be remembering each and every site you have been visited with the details such as date and time of the visit for the last three months that you have been using Firefox!!!. This will be really very very huge if you are a power user like me who spends considerable time online.