I'm the original reporter of the problem. I just noticed that the same hangs occur even with adblock disabled in case multiple applets on the page do LiveConnect operations at the same time but with lower frequency. I stumbled over a blog entry talking about threading issues: http://www.nakov.com/blog/2008/08/24/jsobject-is-not-thread-safe/ but actually it did not help to synchronize the LiveConnect operations in the applets. For this I have not yet a simple test case but I just want to mention it. Maybe it helps finding the bug.
A fellow just noted that the "new plugin architecture" is implemented in a different plugin called jre/lib/i386/libnpjp2.so. Using it instead of the libjavaplugin_oji.so fixes the hang issues. Nevertheless the hanging issue with the legacy - but widely used - plugin is a bug either in the mozilla code base or the JRE.
Same crash (and 100% cpu) problem here. [Firefox 3.5.1, Java 1.6.0_14, Crux Linux 2.5] Comment #6 did the trick. Issue SOLVED, here. Thanks, Ronald Wahl.
Same crash (and 100% cpu) problem here, but Comment #6 didn't solve the issue. Firefox 3.5.2, Java 1.6.0_15-b03, Fedora 11. Tested on facebook image uploader.
Hangs also on Microsoft livemeeting and our company's travel expense reporting tool (java application)
I'm marking this bug as WONTFIX per bug #1269807. For more information see - https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/