Is there a passcode- protected privacy browser for a Mac desktop computer?

Asked by wtf (24) August 17th, 2010 from iPhone

I share a computer with my room mate. It is an old iMac.

On my iPhone I have a privacy browser that requires a login. Is there any way to replicate this on the desktop?

I know I can set no history and no cookies but I am seeking a less obvious way since he sees this and makes jokes assuming I am downloading porn when I just want privacy.

Also please do not suggest a new user profile on the Mac. I am just looking for a browser that a passcode can be set for.