firefox 3.6 not caching css and js files

I have an in house app that has been running for several years, with
regular enhancements. Just recently I have noticed that on windows
clients, firefox 3.6 is no longer caching any css or js pages.

Watching the pages load in firebug, they are all showing as :
304 Not modified
but are re-loading anyway. This does not happen with IE or firefox on
linux or mac.

But firefox seems to be ok with other web sites, so I still have to
suspect something with my setup, but don't really know what to look