Remember Firefox 3.0? It had a function named "clear private data", and you could delete your history with a simple click. In Firefox 3.5, that function was dropped. Why?
Don't worry. It's back! Yep, the same thing that did Firefox 3.0: just press Ctrl+Shift+Del, select Tools > clear recent history or add the Sanitisminau button to the toolbar. Sanitisminau will clear your history WITHOUT asking, so you won't have to see that boring window. Never.
You can select what to erase by going into Tools > Addons, then selecting the button Options next to Sanitisminau.
WARNING! ACHTUNG! ATTENTION! ¡CUIDADO!
* As you can CLEARLY see in the previous text, Firefox WON'T ask you before clearing your history, your cookies and all that stuff. It'll simply clear it!
* Private data are cleared using internal Firefox methods. That methods are totally UNSAFE: your history will still be easily recovered by the FBI and other good guys. In fact, if you're planning to use your computer for evil things, remember: YOU ARE BEING WATCHED -probably.