When I install version 7 of Java on my windows 7 64 bit machine (I install this version 64 bit, windows with SDK) then Java does not launch when entering a website with applet. But when I use version 6 everything works fine.
On the other hand, while compiling (with Netbeans 7.1.1) everything works fine on both of these versions. Java programs also work fine when launched from command line with java command.
What is going on?
EDIT
Now I have tried reinstalling everything with Java 7 update 4 (64bits) but it still does not work. The problem might be somewhere in PATH variable because the java icon does not appear in the tray when I am entering a site with applet but still - I have added C:\Program Files\Java\jre7\bin to PATH and it does not work.
The Java Plugin must be in same hardware specification than the browser.
So if you install Java 64bits, it will run in IE 8 or 9 64bits.
But Firefox is only provided as 32 bits binary for Windows from the standard download channel. You should try with Firefox 64 bits.
Probably your Opera version is 32 bits too...
I invite you to cross-check by installing a 32 bits browser (Firefox standard binaries) and a JDK 32 bits on Windows 7 64 bits. It should work.