Firefox is available for Linux, OSX and Windows. It's open-source to boot and is available in different builds and brands for different systems.
That means that 100% of developers can use Firefox, regardless of the OS they choose to run.
It's also available under Solaris, AIX, *BSD, IRIX, HP-UX...
That makes it very close to 100% of the desktop and laptops around.
The only software I know runs on more platforms is Emacs, which I had on my WinCE-based IBM workpad z-50.
Two browsers can each be good without one rendering the other entirely useless. Safari's development environment is in many ways better than Firefox's, so why not use it when you've got it (which is 99% of the time)?