This answer only applies to Firefox 3.6.14 and Firefox 4 beta builds. In this release Mozilla broke support for applets:
https://bugzilla.mozilla.org/show_bug.cgi?id=629030

This will hopefully be addressed shortly by the Firefox developers, this is outside our control.

You can download the previous version of Firefox here, which does not have this problem.
http://download.mozilla.org/?product=firefox-3.6.13

Update: Mozilla has released Firefox 3.6.15, which fixes this issue. If you still have problems, please contact us.