Comment 3 for bug 228032

That was using 3.0~b5+nobinonly-0ubuntu3, using sun-java5-jre version 1.5.0-15-0ubuntu1 (java6 fails differently).
There's no addon involved, since I tried with a new firefox account.
That's for the http://impots.gouv.fr website (https://cfspart.impots.gouv.fr/tdir-06/accueil.html I don't know if this will work for you).

If you search the forums you'll find a lot of people having problems with the last part of the process. At the end you need to sign your "declaration", and most people under ubuntu end up seeing two blank rectangle with nothing to click.
I debugged it by launching the java console and it showed me the error I posted, that's because the website sends a .so (libjsec.so) and it can't find some libs.
Since firefox-2 provides the symlink, I see no reason why firefox-3 shouldn't. As a workaround, people can aptitude install libnspr4-dev and libnsss3-dev, which will provide those symlinks too (but globally in /usr/lib, not just for firefox).

And no I can't show you the steps to reproduce, unless you are a french taxpayer (I can't redo it too, since it worked for me).

And sadly, it should work ff2, but since java is broken with ff2+hardy there's no way it can easily work for people (I've seen people advising to add this symlinks by hand in /usr/lib).