It's possible that working on new technologies like WebVR will feed improvements to other parts of the browser (faster parsing, more robust rendering, WebGL, etc). Even if it doesn't have a direct impact, just having more smart people involved in brower tech will help.

Literally nobody will notice when Firefox will start shipping components written in Rust. It will be some footnote about security somewhere that will be ignored. But everyone will be like OMG VIRTUAL REALITY THE FUTURE IS HERE.