Both Go and Jeff Dean's projects are meant for server-side work, so it is unsurprising that Mozilla is not a rival in this space.

However, it turns out they are heavily innovating in the same space as Go with their stunning work on Rust. Additionally, they are building world-class next-gen products with their work on Servo and Daala. Not to mention fantastic tools such as http://rr-project.org/.

I believe your assessment of Mozilla's engineering talent is inaccurate.