Reasons:
1. Firefox is losing.
2. With Apple moving to Swift, Google needs a better language than Go to replace Java on Android, but can not wait 4 years. So Mozilla's Rust is good choice second to none.
Would you really need to buy Mozilla to use Rust lang ? For App development, isn't Go a nicer language then Java ? And isn't that enough ? Firefox, just released Firefox OS. It's been my primary Browser under Ubuntu and Mac OSX.
Go is not good enough at expression. Firefox's market is dropping.