Chrome and IE cache static content, Firefox does not

I am trying to improve the performance of my website by adding cache headers for static content.

So far, I can get the content to cache in Chrome and Internet Explorer, but not Firefox.

Here are the caching-related headers I'm supplying:

Cache-Control:private, max-age=1800

ETag:"809067e0179acb1:0"

Expires:Mon, 20 Dec 2010 21:35:10 GMT

(NOTE: 开发者_JS百科ETag and Expires are variable; Expires is 30 minutes in the future)

I verified the behavior using Fiddler 2. Chrome and IE7 do not request the images, CSS, and JS after the first request, while Firefox requests them every time.

Is there any header I should supply to make Firefox cache these?

UPDATE 2010.12.22

I noticed the same behavior on most websites, including www.yahoo.com. Is there a way to force Firefox to cache?

This would have occurred for SSL content, but it should no longer be the case. FireFox altered the way it caches with the resolution to Gecko bug 531801.

Now, SSL content is cached to disk regardless of the Cache-Control header.