After some minor troubles for uncompatible extensions and themes, I definitively passed to the brand new release of Firefox and... damn, it's unbelievably faster and nimbler!!!
For me there were only a few things to fix.
The first is that some extensions do not work properly on ubuntu since it has a modified version of Firefox. But for me it was enough to install the xulrunner-1.9-dev package from the standard repositories, as suggested by mkalen.
The second is related to the checkboxes and radio buttons rendering under KDE 3.5. For me the solution was to install the gtk2-engines-qtcurve package and then choosing not to use the KDE style for GTK applications. For that, you have to go to the GTK styles and font sub-section of the Appearance section of the System Settings, where you have to choose use another style and select the QtCurve style.
After that, I can say that Firefox3 is really the very best browser yet.
