A technical reason for GNU/Linux not to use Rust or Go is that C supports a lot more platforms so you can't replace core components at this moment, also I want to remind you that memory safe languages existed before Go and Rust and the only project I am aware to create a safe OS and utilities was Midori by Microsoft.
Firefox and it's dependencies is not yet 100% Rust so I honestly expect a new browser started from scratch in a safe language to be done before Firefox is "ported".