header(‚ÄúCache-Control: no-cache, must-revalidate‚Äù)
header(‚ÄúExpires: Mon, 26 Jul 1997 05:00:00 GMT‚Äù)
So IE was using the server-side cache, while Firefox was not. Weird. Must be because IE handles it more like an object and FF handles it like a page (just a guess).
Moral of the story: include the Cache-Control stuff or else‚Ä¶