How To Prevent Firefox 5 From Storing History Of Visited Websites
Here is how to prevent Firefox from remembering your website history :
1. Select Tools > Options.
2. Under Privacy tab, select Never remember history and click Ok.
From now on, Firefox 5 will stop remembering your visited websites list.
By the way, if you like using add-ons, here is a cool add-on that does the above and some more.
Cheers.