You could consider instead setting up a separate firefox profile for your friend, better still why not set up a separate Operating System account for other users on Windows XP.

I'm not very computer savy,how do I set up another windows operating system?

Pressing the function key F1 after you have started up and before you open firefox or any other applications will get the XP help and support info, alternatively use Start -> Help and Support

You could also look at this article; click this link "How to create and configure user accounts in Windows XP"

By setting up multiple user accounts each user may alter their own settings etc without causing unexpected changes to other users. In some instances the additional user accounts may be restricted so as to reduce what the user is allowed to do. There will be many articles you can read on the web, this is one official MS document User Accounts overview

See also:

• Public Fox: https://addons.mozilla.org/firefox/addon/3911

Remember that it is an extension and can easily be bypassed by starting Firefox in Safe mode