I've used Crossover Mac on occasion. I even bought it because I like the idea of what these guys are doing.
It's cute for what it does: translate Windows GUI to native GUI (Mac in this case).
Keep in mind, though, it *is* a translation, and details often get lost. You say you want to check Firefox. Why not use Firefox for MacOS? One reason could be that ports are not always complete. Well, neither are translations. If you test your website using MacOS Firefox or Crossover-ed Windows Firefox, there's no real guarantee it will work the same way on genuine Windows Firefox.
It would probably be best to test using BootCamp or VMWare or Parallels. Of course, for those you still need a licensed copy of Windows.
MacWorld had a look at various Windows-on-the-Mac options recently, but it's already out-of-date (VMWare is now beta 3).
Macworld: Feature: Four ways to Windows, Page 1
Personally, I use VMWare Fusion for my MacWindows. It's going to be a great product when released. Definitely worth buying.
HTH