The browser will only obay what the server sends. If the server doesnt send an explicit expires time then the browser is left to guess, and it seems firefox/mozilla are more likly to - same for proxys along the way.
Im stuck on a 256k internet connection here because its the only one available in NZ without rediciously low transfer quotas before the speed plummits to an un-usable 64k, as the latency is high coming across half the globe even the faster connections dont really speed up page load times from forums like this.
Avatars are dealt with by turning them off

its inlined images that are the killer like in the project threads