To be honest, it's not possible to maximize a firefox window completely on a mac. However, it is possible to maximize the window to fit in the whole screen. To do this just follow these steps:
1. Create a new bookmark by right clicking on the bookmark toolbar. If you don't see the bookmark toolbar just go to View > Toolbars > check Bookmark toolbar.
2. Give the bookmark a name like Fullscreen (something that you can remember) and then paste the code below in the Location box. Make sure you paste all the code in 1 line only though, or else it won't work.
javascript:self.moveTo(0,0); self.resizeTo(screen.availWidth,screen.availHeight);