Servo currently uses SpiderMonkey as its JavaScript engine (SpiderMonkey is the Firefox JS engine). Sure, a JavaScript engine written at Rust would be useful at some point, but right now that's not on their roadmap.
Stuff like JITs can't really reap Rust's safety+performance benefits[1], and overall the same might be said of a JS interpreter, with all the garbage collection and stuff. The other thing is that Spidermonkey/Chakra/V8 have had years of optimization and tweaks, starting from a clean slate would be _very_ hard. With Servo's small team this is an impossible task, but with a larger, dedicated, team, there's a chance. shrugs
[1]: though it's possible they will still be safer than the C++ counterparts, which is still good. The question then becomes, how much is that increase in safety, and can it justify a whole rewrite?