I did. Everything's fine except an add-on that seemed to be incompatible, but I can't remember what it was so after all it should have been something almost never used. About apparent no changes from FF4, it could be that they are trying to give the sense of innovation that (strangely) is given by the IE versions marketing. I don't bother if the interface looks the same, it is not the way I judge software major releases. I suppose there must be something changed in the core, and maybe that was already almost ready when FF4 shipped. In the worst case, there must be at least security enhancements. And since it took me almost no time and no efforts to upgrade, why not?