I have a "Refresh" button on my PHP page that uses Javascrip (window.location.reload()) so the user can update the page.
Unfortunately the page includes a number Javascript and CSS libraries (eg. jQuery) that get reloaded too with this kind of refresh.
The libraries are servered with a far future Expires: header and when the user simply requests the main URL, Firefox doesn't reload the libs. But with the refresh button, Firefox sends a "Cach-Control: max-age: 0" header and all libs get reloaded.