First, Rust is a handful of people working on a research project.
Electrolysis was "put on hold" because there were better ways to make Firefox a more responsive browser, now. That's projects snappy and supersnappy. See https://wiki.mozilla.org/Performance/Snappy and http://taras.glek.net/ and https://bugzilla.mozilla.org/show_bug.cgi?id=718121 -- the final bug being what I think will make Firefox a great browser again.