I've been a manager at Mozilla for several years now. I became a de facto manager by necessity, when we established the Auckland office, and became a de jure manager a little later when the increasing size of the platform team meant we couldn't all report to the same manager. (It's hard now to remember what those days were like!) I've never been passionate about being a people manager, and I've always looked to reduce or eliminate that part of my role, but our pace of hiring always outstripped my ability to send my direct reports to other managers. Lately that's changed, partly because our hiring has slowed in the areas I touch, but also because we have more and better managers in the platform team to take up load. So, the people-management side of my role is finally shrinking. I think this is good for me and Mozilla --- I think my time and attention best serve Mozilla when used for tasks other than people-management.
This means if you're looking for someone responsible for running a Gecko platform team, it's probably not me anymore. For layout, it's Jet and David Baron, for graphics it's Jet and Milan, for media it's Anthony Jones, and for WebRTC it's Maire. Those areas also have their own technical decision-makers.
My decreasing management responsibilities have been offset by increasing responsibilities for leadership in other ways. As Mozilla has grown in size and scope, and my tenure lengthens, I've been playing the "senior figure" role more and more. Sometimes this is nothing more than repeating what a lot of engineers know, but putting my status behind it. Sometimes it's about drawing on broad knowledge of Mozilla and the Web platform to instruct people. Sometimes it means trying to modify or review code no-one else will touch :-). This is all necessary and important, but I'm not completely comfortable in it yet. Sometimes I'm hit by imposter syndrome. Most of the time I'd rather be writing code :-).