If that is happening just once a day, I would suspect that Firefox is updating the '''urlclassifier3.sqlite''' file when that happens. That is the Safe Browsing - Anti-Phishing / Malicious Websites database file that is updated once a day = it is usually updated the first time Firefox is used each day. http://kb.mozillazine.org/Urlclassifier2.sqlite ''older version of that file'' Deleting that file, which forces Firefox to create a new version of that file, may work to make the daily updating of that files less system intensive or noticeable. A fresh load of that data will result in a much smaller file for Firefox to deal with. ''Data never seems to be removed by that file, only added. And it can get to be quite large. I have seen it as large as 67MB - on my system the lag seems to start when that file is about 60MB.'' https://bugzilla.mozilla.org/show_bug.cgi?id=383031