I have to say that generally I prefer Apple Safari for day-to-day web browsing. But Firefox also has a place on my Mac, in fact, a very important one. Without Firefox installed, a few of the web sites I need to work with won't function correctly.
Even though I prefer Safari, Firefox is one of the best available browsers for the Mac, and Mozilla is very good at keeping it up to date.
It seems many other Mac users agree that Firefox is indispensable, so much so that Firefox won the Reader's Choice award in 2010 in the web browser category.
If you need Firefox, you can download the Mac version from the Mozilla web site.