Regarding BROWSER DIVERSITY STARTS WITH US.

I've run across more sites that don't work with Safari than don't work with Firefox. While Chrome's engine, Blink, is a fork of WebKit (Safari's engine), that schism happened in 2013 and the two have basically speciated at this point. Zeldman isn't wrong that people should use Firefox but he is leaving out, for some reason, Safari as an acceptable alternative to Chrome.

IMO, if you are on a Mac you should use both Firefox and Safari. I primarily use Safari, but backup to Firefox when it doesn't work with something.