There are already tried Firefox 4 Beta?

Yes, I have. The first thing that I noticed is its similarity to the Opera interface. The difference is the existence of add-ons. One Firefox fanatic accused Opera as the copy cat. I have been using Opera since version 5 about 10 years ago, and Firefox which came out years later, so I do really know what went on. Satisfactory? Not quite. Some of the problems that existed in the previous versions have not been completely ironed out. In a nutshell there should be major changes or improvements before I make some consideration in changing my default browser to Firefox.