Quantum Compositor (an architectural improvement written in C++) just shipped.
Quantum Render/Webrender (Servo's pure rust renderer that uses the GPU a lot) is in nightly, off by default (there's an about:config prefer, though it may not work depending on your drivers. about:support has more info). The current version of quantum render actually kind of renders twice -- first using webrender and then the regular one to fill in the missing bits. So don't expect it to be fast just yet.
Quantum Style (Stylo), servo's pure rust styling system, can be enabled at build time. We're discussing making it a preference in nightly, there are a couple of blockers for that.
The Rust URL parser (rust-url) can be enabled via network.standard-url.enable-rust. This will parse URLs using both rust-url and the regular parser, and complain about differences. I believe it's on by default in nightly and Aurora. It will fall back to the result of the regular parser if there is a difference, though there's a build time option to disable this. There are still some discrepancies which need fixing (most of them are actually gecko side bugs, we've fixed most if not all of the rust-url side spec issues), and this project is on the back burner for a bit since I'm busy with Stylo, but I hope to get back to pushing this soon!
On nightly, regardless of prefs, IPV6 URLs will always be serialized using rust-url. There were some spec bugs in geckos implementation and we decided to just use the rust stuff since we had bindings for it already. This landed a month ago, and should be in the Firefox 55 release in august.
The MP4 metadata parser in rust has been in Firefox releases for a while now.