If there is a Java applet embedded, you would typically have references to '
.class' files on the page.
If you use the firefox addon NoScript Java launch will be blocked.
If they get blocked, you can make NoScript permit execution of the scripts that launch the applet
(typically with the status bar icon )
If you have also installed the Adblock Plus addon, clicking on its Icon () in the Firefox menu will open a blockable items window in the browser. This will show all the class files that are allowed.
You can again explicitly block specific Java applets here.
Update: You can limit to just the AdBlock Plus addon and click on the red icon to see if there are any java files linked.
Further on your comment in the question.
- do they see another browser also crash similarly for the same sites? (IE, Chrome)
- have you tried for the negative-case where a Java based site does-not crash the firefox?
- does this firefox in question have many other extensions installed (maybe, experimental ones)?
Update2: on your secondary comments.
If its detection of Java that interests you. Installing just the AdBlock Plus addon
should suffice. You could always just open the "blockable items on current page" while on the site and type "class" in the Search bar there.