One of the cool new features of the upcoming Firefox 3.1 is Private Browsing. Essentially, when you enable Private Browsing, nothing of what you do in Firefox is recorded. Great when you want to hide something from a nosy flatmate. So in Ubiquity fashion, here's a command to easily toggle Private Browsing mode.
Once you've subscribed to that command, all you need to do is open Ubiquity, and start typing "private-browsing". Typing "p" may even be enough, depending on the commands you have. By default, it will toggle Private Browsing to the opposite of the state its currently in. You can also explicitly specify whether you want it on or off by specifying "on" or "off" respectively. That way you don't have to actually remember or check whether you're already in Private Browsing.
The command can be subscribed to from here:
Note: You'll need to be running a nightly build of Firefox 3.1, or 3.1 beta 2 (when it gets released) or newer.