Mozilla Firefox is one of the most popular web browser on the Planet, it’s a free and open source browser, available for all platforms. In Ubuntu 12.04 LTS (Precise Pangolin), Firefox comes installed by default (it’s also true for most GNU/Linux distributions).
In last few years – Google Chrome has also become very popular.
Being the only closed source mainstream browser isn't a good position.
In the world of mainstream Web browsers, Microsoft stands alone. Apple's Safari browser is a closed source shell wrapped around an open source rendering engine, WebKit. Google's Chrome browser is a closed source fork of an open source shell wrapped around an open source rendering engine, Blink.