i am using FF and to get java to work and change my useragent with to have java and it still work I was able to make a site using Microsoft IIS 6.0 crash.. it was a useragent site showing one's current useragent.
the site in question was/is http://www.fiddlertool.com/useragent.aspx
i'm serious - i wasn't even trying and all i was doing is what it showed/got from my new useragent but anyway in the process now I know what makes java crash FF, it's this part:
if u remove the 5.0 and leave it blank - java will crash the browser upon executing, however u can change/put a diff. # there or letter, etc.. and java should work fine..