Opera's got the best interface, and unlike firefox things dont "pop out" of the main window.
The download manager, the mail client, the widgets, every thing is neatly integrated.
It supports torrents. Its skinable. Over the years it has been definatively proven to the the lightest, fastest, neatest and securest (yes even more secure than firefox) browser.
Its acid 2 compliant.
It comes with a built in spell checker.
To top it off, there is a J2ME version for mobile phones (its uber cool, try it even if you dont have GPRS activvated!)
Importantly, it introduced tabbed browsing and mouse guestures truly boost you efficiency.
Opera is definately #1, it is the smartest, coolest, safest, fastest and a whole bunch of other superlatives.
And it beats firefox.
End of story.
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