Handling null related to Java objects is not working correctly inside Javascript under some scenarios . Different browsers behave differently for this scenario.
Consider a scenario where a file object received from Java applet and "null" check on this object while doing a readLine() is not working with new Plug-in.
Try running the following testcases which tries to read line from a file(contains 2 lines) inside the javascript
http://sqeweb.sfbay.sun.com/deployment2/jitu/plug-bug/ojiliveconnect/src/LocalFileAccessTest.html
After reading 2 lines from the files following condition should be met but that's not true in case of new Plug-in:
while ((line = fileRead.readLine()) != null)

On IE in case of null empty string is getting passed while on FF3 it's getting passed as "NPObject JS wrapper class"