I don't think IE9's abandonment of XP is going to slow the adoption of new web technology all that much, something that Mozilla seems to suggest. There is a lot of momentum behind them with mobile devices supporting them so well. Still, 40% of web users, even if those are among the lowest value web users, is worth developing around.
Mozilla isn't around to make a profit, so spending on XP users makes more sense. Abandoning them would have also made sense.