I think it depends on whether FF got rid of 6.0.19 properly. I know it took a couple of tries with me (just had a look at my add ons and 6.0.19 is still there. Just disabled it too see if that helps). Mind you 6.0.20 could be a little unstable as well. You know how it is, you do something to make it better and all you do is introduce a whole shit load of new bugs or is that just m$?