#sanitizeSeparator, #privateBrowsingItem, #sanitizeItem  
 #privateBrowsingAutoStart  

1. In case Firefox deletes history items automatically: Check Firefox settings about History remembering and deleting: Tools->Options->Privacy->History Remember my browsing history for at least: Clear History when Firefox closes 2. In case you sometimes allow other person to the computer and suppose this person deletes traces of illegal websites: 2a. Go to page "about:cache"(without quotes), it must look like this: Memory cache device Number of entries: number Maximum storage size: number KiB Storage in use: number KiB Inactive storage: number KiB [file://pathtomemorycache List Cache Entries] Disk cache device Number of entries: number Maximum storage size: number KiB Storage in use: number KiB Cache Directory: Profile Directory\Cache [file://pathtodiskcache List Cache Entries] Offline cache device Number of entries: number Maximum storage size: number KiB Storage in use: number KiB Cache Directory: Profile Directory\OfflineCache [file://pathtoofflinecache List Cache Entries] Click any of the three links on your actual page, and you will see some recently visited sites. However, the entries here might also be deleted if the person in question asks Firefox to "Forget about this site". 2b. There is a program [http://www.blogsdna.com/4252/recover-deleted-firefox-35-history-with-firefox-history-recover.htm Firefox 3 history recover]. It requires some technical skills but the result is great: you can recover the deleted parts of history. [http://sourceforge.net/projects/ff3hr/ This open-source tool] actually works like a forensics tool and only a very technical person could erase a file in a way which couldn't be recovered by such a program. I hope that you will be able to cope with this program despite its command-line interface. 3. In case you sometimes allow other person to the computer and suppose this person uses Private Browsing to leave no traces of illegal websites: In Private Browsing no history is saved. So the only thing you can try is to prevent person from going to Private Browsing. You can hide the Menu items for turning Private Browsing on and off, but the keyboard command will still turn it on: Add this code to userChrome.css below the @namespace line. See [http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files] #sanitizeSeparator, #privateBrowsingItem, #sanitizeItem #privateBrowsingAutoStart You can provide feedback about the Private Browsing feature to Mozilla here: [http://hendrix.mozilla.org/] Maybe if enough people request a "hidden switch" to turn of that feature, they might do something about it. 4. Whatever browser the person uses, you can still try to log the websites visited. If you are administrator of the computer, go to Windows Firewall and enable logging of IP addresses and other data about connections to the computer. 5. My personal suggestions: 5a. Don't allow this person to use your computer. Illegal sites can bring viruses. Your computer isn't a bastion to withstand all of them. 5b. Set up a good firewall, which stops all bad websites from connecting to your computer. Then the person will not get what he wants and will have to leave your computer.