Rust is definitely a significant and important project. That said, Firefox's peak market share was a little over 30% of desktop browsers (in 2009 so they weren't yet a sideshow), while Rust isn't even currently in the TIOBE Index top 20 (can't find any easy way to see a peak position sadly).

More importantly though, we have other Rusts. Rust has half a dozen competitors and essentially nobody thinks that making a robust new systems programming language is an unachievable goal. Firefox is the only non-profit browser engine we have and most people consider it an impossible undertaking to develop a new one from scratch at this stage. Indeed, Microsoft recently tried before conceding defeat and switching Edge over to Chromium's Blink.