I primarily use Safari, but I use Firefox whenever a site doesn't work in Safari, or I need to see stuff accurately for web design stuff. On the mac, Safari is faster than Firefox, so that's why I use Safari as my default browser. Also, being an RSS aggregator and reader allowing me to open as many rss feeds as i want each with their own search filter in one window is just downright awesome XD
I used to use Netscape, until 7.0 became unusably buggy on my computer, so I switched to IE. As time went on, it became a pain in the ass, because of it's inability to work the way I needed it to, so I tried Safari. I haven't looked back since.