Why not now? One thing that I love about Firefox is its address bar implementation -- I find it so much better for finding old sites that I've visited before than other browsers. Just type one or more fragments of the URL and/or page title and 99% of the time it's right there in the suggestions. Incredibly helpful for finding bugs in Bugzilla, among other things. With Safari and Chrome I find it's never as smooth or easy.
And don't forget the add-ons. As well as the well-known ones like AdBlock Plus, I recommend "Tree Style Tabs", which makes organising large numbers of tabs much easier, and "It's All Text", which lets you user the editor of your choice to edit any textbox in a web page. See https://addons.mozilla.org/en-US/firefox/ for these add-ons and more.
- doesn't natively interoperate with my keychain (and by extension iCloud Keychain). This is a huge win for using Safari on both my laptop and my iPhone.
- scrolling is still — after years of waiting — not up to par on OS X. No rubber band or proper inertial scrolling.
I keep meaning to give Firefox another try - after ditching it for Safari, and then Chrome, some years ago - but I never quite find the motivation.