I was very critical of Mozilla in the past about all their "science projects" like Rust. While I appreciate Rust, I thought it was wrong for Mozilla to be wasting resources on it, instead of focusing on Firefox. I was so harsh with them only because I strongly believe that Firefox is my only hope for a free web. But then I tried 57, and I must admit, boy was I wrong. This thing is fast! I haven't stopped recommending…
Rust was pretty much designed to solve problems for Firefox. It's IMO the best bet Mozilla has made these last few years and we're just starting to see them reaping the rewards.
It seems at some point they became aware of how unsuited C++ is for parallel processing, and how important it would be for browsers in the future. Even compartmentalization or running tabs in their own process is a form of parallelization. This requires an efficient way of sharing memory and preventing race conditions and errant writes, as well as a way to protect these processes from cross-site scripting attacks, buffer over- and underruns, etc.
Computing hardware has also moved on from the CPU-centric world and the gains in battery life from dedicated GPU hardware being used even for rendering tasks, and now for font processing and other things were too good to pass up.
I'm a long time follower of Mozilla's development, and extremely excited about what's happening with Firefox.
(I'm also currently a Chrome user, leaving in large part due to profile corruption issues and inferior built-in development tools. I hope to be able to run Firefox and WebRender on both Ubuntu and Windows.)