Firefox not using cached JS Files

I'm not really sure if this is a firefox problem or a Rails related
issue, but I thought I'd ask to see if anyone has had a similar
problem. When I access my application in IE all of the default JS
files seem to get cached after the first page view and then loads
happen very quickly. However, in FF all the JS files appear to be
loaded every time, yet they appear in my about:cache. Here is an
example entry:

     Data size: 30550 bytes
   Fetch count: 13
Last modified: 2007-01-03 23:15:13
       Expires: 1969-12-31 18:00:00

The expires seems weird to me, but I don't really know what to look
for. Has anyone else seen this? Here's a link to my site, maybe it's
due to railsplayground?