I cannot stress enough how much this bothers me! - I spend days / weeks / months getting a site FF/I.E compliant etc. Yet 1 visitor who look at my page for 1 second may visit once a month using FF..
Lets just say in 5 years time everyone should use 1 browser; the development side of things would be such a happier place :)
I've never had not many problems making a site that works in both FF and IE. Either you are using IE proprietary markup or your design methods needs to be reexamined if you are having to spend that much time creating a site that is compatible with both browsers.
And the last thing we need is there to be only one browser. That just encourages stagnation from both the user and developer ends.