This sounds like a Javascript issue, rather than Java. For all the two have similar names, and similar syntax, they are not the same thing.
I'd try disabling any (all?) Add-ons, just in case one of those is buggering up things, then try uninstalling Firefox, and delete any remnants it may have left behind, and then reinstall (remembering to backup things like Bookmarks obviously). If it remembers things like bookmarks and home pages etc, then you haven't been efficient enough in deleting stuff!