I don't think Servo will "land in Firefox" (it's possible, but it's a lot of extra work). However, components written in Rust (taken from Servo or written in scratch) may indeed land in Firefox -- there are a couple in Firefox today! (Except on Windows).

https://wiki.mozilla.org/Oxidation

(There are a bunch of alternative paths forward other than sharing components with Gecko that don't involve dropping it into Firefox)