Why does FireFox 3.6.8 not cache static contents from asp.net developer server?

Question

I am working on a asp.net web site, like normal user, we use asp.net developer server during coding and testing. Today, I found the firefox not cache any static file of my site, since our application is pretty big, it made page load time very slow. I checked firefox about:cache, all the static file cache setting looks like

           Key: http://localhost:26851/App_Layout/icons/actions/email/folder.png
     Data size: 871 bytes
   Fetch count: 1
 Last modified: 2010-08-19 11:59:46
       Expires: 1969-12-31 16:00:00

           Key: http://localhost:26851/Framework/ScriptLibrary/JQueryPlugins/ui.mouse.js
     Data size: 5079 bytes
   Fetch count: 1
 Last modified: 2010-08-19 11:59:39
       Expires: 1969-12-31 16:00:00

FireBug shows header like this

Server  ASP.NET Development Server/9.0.0.0
Date    Thu, 19 Aug 2010 22:10:27 GMT
X-AspNet-Version    2.0.50727
Cache-Control   public
Etag    "1CB3F32C834A880"
Content-Type    text/css
Content-Length  1775
Connection  Close

Firebug has another tab called "cache", the information is:

Last Modified   Thu Aug 19 2010 15:10:27 GMT-0700 (Pacific Daylight Time)
Last Fetched    Thu Aug 19 2010 15:10:27 GMT-0700 (Pacific Daylight Time)
Expires Wed Dec 31 1969 16:00:00 GMT-0800 (Pacific Standard Time)
Data Size   1775
Fetch Count 10
Device  disk

The expires date is set back to 1969-12-31, I believe that's the reason why they are loaded very time. I am using Visual studio 2008, windows 7 machine. The application works fine in IE, the contents are properly cached.

Did anybody see this behavior before?

Answer 1

You need to issue Expires header to make Firefox cache the files.

Otherwise, how do you think it could guess for how long does it need to cache the files?