Can Firefox 4 and Firefox 3.6.15 coexist on the same OSX machine?
Yes, you can have both versions. In fact you can install the new Firefox in your home directory's Application folder (and it will be only accessible with your account). If you don't have ~/Applications folder, you can create it (and Finder will mark it with the same icon as the /Applications one). Note that you cannot use both versions simultaneously. You can also rename the Firefox.app to Firefox 4.app, and install it in /Applications - it'll work flawlesly.

ps. I prefer the first method - Install applications that I'm testing in my home Applications folder, and when I'm sure they're ok, move them/replace application in /Applications.