I'm having a strange problem with my CSS stylesheets. I'll go to my site in Firefox (2.0+)/Mac and leave the browser window open for a long time. Then an hour later I'll go back to the window and navigate to any other page in my site. When the page is loaded, none of the stylesheets are applied and the whole site falls apart. Not until I empty cache and refresh does the stylesheet come back. It's typically not a problem if I just start browsing my site as soon as I launch Firefox.
Any ideas why this happens and what is the fix?