If you're building Firefox from source, please check that your rust
toolchain is up-to-date. You can either:
rustup update
or
./mach bootstrap
We've just queued a patch to bump the minimum required rust version from
1.15.1 to 1.17.0, the version released 8 weeks ago. This is necessary for
merging recent changes to the style system from servo as part of the
Quantum project.
The commands above should update you to at least 1.18.0, which is even
better. See
https://bugzilla.mozilla.org/show_bug.cgi?id=1374807 and
adjacent bugs for details.