I need to use some sensitive websites (personal mails, banks, etc.) in Firefox in a public place.
How can I obscure the screen content so that people passing by wouldn't immediately know that I am on, say, Citibank site, or GMail?
I would prefer a solution for Firefox, but non-FF solutions are also welcome. For example, if I can obscure a particular windows on Windows 7 (say by turning it into negative image) I guess that would be cool too.