Because Firefox is an open source, other browsers can be based on it. It can also be ported to systems that Mozilla don't support (or have dropped support for), and it can be optimized to specific systems.
Go to http://www.palemoon.org/ for a Windows version, optimized for modern processors.
http://www.riscos.info/index.php/Mozilla_Firefox RISC OS version (based on Firefox 2)
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Browse with Firefox and Opera