Under what conditions does firefox honor cache expires / headers

Firefox is caching 10sec 1.5MB videos but not 50sec 8.5MB videos. I assume its because of the file size, but I'm not sure. Under what conditions does firefox honor caching requests?

I'm using this code to force caching of webm files:

<FilesMatch "\.(webm|ogg|mp4)$">
Header set Expires "Mon, 27 Mar 2038 13:33:37 GMT"
Header set Cache-Control "max-age=864000"
</FilesMatch>
AddType video/webm .webm

There's two different questions here. HTTP cache headers can never specify that a user-agent must cache a resource, only that it must not cache a resource. So Firefox, by not caching your video, is not failing to honour the headers.

However, Firefox clearly has some rules about when to cache resources. I don't know what they are, but I recommend that you don't rely on them. They are likely to depend on the platform on which Firefox is running in quite intricate ways.