it's not much of being IE centric as to designing an api first and insteaad of losing time on stupid JS/DOM dependancies (things you can fix later for less cost), just get it maybe to most users for whom IE is the internet.
IIRC there are more IE related JS/DOM bugs than Firefox ones. In FF you also have Firebug which makes development less painful. So why would it help to make an IE version only first? The only reason I can think is if you're using ASP.NET and the funky HTML generation that it and Visual Studio do.
of what i know of IE suffers from certain memory leakage issues and that alone makes you write more careful JS code, that is if you are aware of them. So, if you can avoid these and achieve satisfactory functionality between all recent versions of IE (especially when putting these on their limits) , you can later fix your code working for Mozzila. Of course you could do it simultaniously but would it worth to lose even more time in cotrast of going online. (not a .NET. fan)
If you think your audience is primarily IE users than you can make a decision to just support IE.
But you asked Hacker News. For the people here I'm sure IE is a last choice browser.
Your assumption that it costs less to fix later is incorrect. It will probably cost you more.
depends on what you consider cost. Cost to fix or cost in numbers of users.