The problem is FireFox 3
Yours isn't the first report that FF3 breaks the HabitableZone, and a whole lot of other sites besides. Welcome to Nightmare #1 for Web designers.
I plan to install FF3 in the next couple of days to take a look. That's another, lesser, nightmare for people in my line of work, because once I install FF3, I'll lose the ability to check my designs for compatibility with FF2. You just can't win.