Being an open source product, Firefox gives determined developers the opportunity to create a custom version of it for their specific taste or platform.

Neil Lee, maintains special Firefox builds optimized for Mac OS on G4, G5 and the most recent Intel platforms. Version 2.0.0.1 is already available with Firefox or Aqua widgets (form elements and controls) versions. Note that this version is not branded as Firefox because it is not provided by Mozilla, so you get a plain globe icon with no firefox around it. According to users’ comments the improvement is noticeable, specially for the Intel platform.

There is even another effort to provide G5 (Mac OS) optimized builds provided by drumsticks.

Remember OS/2? Well, some people definitely does and love it. So much, that there is an official project for Mozilla products port for OS/2 Warp and eComStation called Warpzilla. Firefox, SeaMonkey, Thunderbird, are all available and up to date for OS/2 users.

RISC OS also has its port and is kept up to date by Peter Naulls. You can check it out at RISCOS.info.

Finally, Bezilla, is a port for BeOS maintained by the Bezilla Team. It includes a port of Gnash, (an open source Flash implementation), so BeOS users can get even some support for Adobe Flash presentations and applications. You can get it at BeBits.