A bunch of people here seem to be wondering whether this will mean a substantial change to Mozilla's strategy. This seems very unlikely – Brendan has been on Mozilla's steering committee for many years, and involved in many key decisions in the past year. I expect him becoming CEO means more of the same – if you like what Mozilla is doing, then you should be happy, and if you wish it was doing something else then you should probably look elsewhere.
It means pigs will fly before Firefox supports alternative languages like Dart or Typescript, since JavaScript is his baby. I'd be surprised if asm.js even continues to be supported, he's argued so strenuously against the idea of javascript as assembly, even though there are now almost 100 other languages that compile to javascript: https://github.com/jashkenas/coffee-script/wiki/List-of-lang... and Google, Microsoft, and Facebook (hack) are all sticking to languages that use types.
He has always very actively supported ASM.js. He's also stepping away from involvement in JavaScript standardization efforts so he can focus on his new position, so this will actually decrease his involvement in those specific areas. None of that probably helps native Dart support at all – there's probably close to zero champions for that sort of thing at Mozilla regardless of Brendan.
