Before visiting a website, use Firefox’s Private Browsing (similar to Internet Explorer’s InPrivate Browsing) which tells the browser not to remember anything you do from that point until you close the browser.
This can be accessed by going to the Tools menu and selecting Start Private Browsing.
If you have already started browsing, you can clear your history by:
Going to the Tools menu.
Select Clear Recent History.
Under Time Range to Clear, select Everything.
Make sure all checkbox items that have now appeared in the Details area are ticked.
Click Clear Now.