My main is Safari, but I have Firefox (Mac) and use it if I need to/if I want to check something.

Um, I know that back when I was using a PowerPC Mac, Firefox was pretty different on my Mac than it was on my PC. I'm really not sure how close they are now, but you may want to look into checking if you should treat them as two different browsers (or three if you're considering Linux?). But like I said, I'm not sure if this is still the case.

Also of note, I run some outdated IE in a Windows environment when I want to get on a website that's super annoying and won't support anything but.