[kenjackson]

Why would the IE team be more resource constrained than the Firefox team

Because they have fewer people working on the product? I don't know. But presumably they have some constraints on resources that prevent them from doing everything -- hence having to prioritize.

Surely that perception hurts them more than the cost of a few developer-months to take one of their existing spellcheckers and drop it into IE?

So then what do you cut? Dev/test time doesn't just appear magically. Do you cut flexbox? Text shadow? History?