I'm sure they want to replace Gecko with Servo but they can't because it doesn't have xul/xpcom and it never will. There was a post a while back about a side project called Firefox.html which is to rewrite the interface in html5 instead of xul.

Anyways they shouldn't replace gecko but instead make a new browser with it's own branding like the Internet Explorer team is doing with Spartan. Because no matter what Mozilla does with Firefox it will never gain market share because they've lost momentum and don't have resources it takes to brute force users to their product like the big guys have.