I think the Reset Safari option in that same menu is even easier (and more comprehensive if you don't know what you're doing). Both of these options, as far as I know, only affect the current user. If you have multiple users each one must do this for their own account.
In Firefox 1.5.x or greater (I think) you can go to the privacy preferences pane and choose the settings option at the bottom and select everything, then use the "clear now" button in the preference pane.
Alternatively, if you're selling or otherwise liquidating it, you can just reformat the hard drive with the install discs.