Safari is my favorite on the Mac since it's quite lean (memory wise, CPU wise and energy consumption wise) and responsive. But I miss extensions from Firefox and don't use Safari if I have to work with several tabs and windows. There are several little things that make using Firefox much better for my experience.

Firefox is my favorite browser overall (especially on other OSes), considering its smaller memory footprint and lack of sluggishness with tons of tabs (compared to Chrome) and the fact that it's from Mozilla, a company I hugely respect and admire (even with some of the missteps that have happened in some aspects in the past few years).

Safari is great, but doesn't have a great solution for vertical or tree-style tabs, which are heavily used in my workflow.

FF is for work, Safari for non-work. Chrome when I need Flash or testing Chrome-only content (some great plugins) - pretty rare.

And battery life.