Username or EmailPassword
To speak to OS integration, I use Opera precisely because if fits perfectly into my KDE desktop, right down to correctly inheriting transparent menus. I find the user interface to be perfect. The only down side is stability. It crashes more than all the other browsers I use. To rate them from most to least stable:
So while I continue to user Opera due to its perfect integration with KDE and superior features to Konqueror, I sometimes switch to Firefox when Opera crashes.