JAVA applets are not displaying

Hi all I have a weird problem. I used portable firefox since v3.0.19 on the same machine at work and before version v3.6.24 I have used many other versions and all of them worked flawlessly (currently I am on v3.6.23 and there is no problems at all).

Recently I've tried to use v3.6.24 and v8.0.1 (while ago I also tried v7) and none of them works with JAVA applets. When I try the same website which works on the older browsers, this new ones just display the source code of the html site containing the applet (just a plain text like in a notepad).

I have the newest Java installed 6 update 29 and it works on old ones. What is the problem with the newer firefoxes that they don't work???