If you don't want your girlfriend to know what you're watching online, you can use Firefox profiles to clear your footsteps. Here's how:
Start by printing this page, because you'll have to close Firefox in order for this to work. Next, launch Firefox with the -p option. If you're in Ubuntu, you can do this by pressing Alt-F2 and typing firefox -p at the "Run Application" window.
The profiles window will appear with only one profile called "default". Create a new one (let's call it private), select it and start Firefox. Next, go to "Edit", "Preferences" and in the "Privacy" tab untick all the "History" options. Tick the "Always clear my private data when I close Firefox", click the "Settings" button and tick all the boxes. Your private profile is now ready!
Now close Firefox and open it again with the -p option. Select the "default" profile and set the "don't ask me on startup" option. Run Firefox and make sure you have the old profile back (with history and all). Of course I'm assuming that's what you want by default.
From now on, each time you want to navigate to websites you don't want your girlfriend to know about (say, because you want to surprise her with a valentines gift), just execute Firefox by typing firefox -p private.