Last I seriously used Firefox (I don't web browse on my Mac), I used Greasemonkey with a couple of scripts, and a skin, and a few other ones. Nothing really major. If I really had extensions that made browsing that much better, I would be using Firefox on Windows. As it stands, it is a nice feature, and there are some useful extensions, but none that change the game so much that I'd make Firefox my browser of choice.
RE: Safari. Safari is just as proprietary in its handling of HTML as IE is, which is one of the big Anti-IE bullet points that Firefox people use. This is why Safari renders so many pages incorrectly--many more than IE does. I am sure Safari got some props because it supported tabs before IE did, back when they were still a hot cutting edge feature.