Per bug 1314071, we should add the dual Apache 2.0 / MIT license to about:license. It is already in use by a number of vendored Rust components in the third_party/rust directory.
I commented in bug 1314071: "Well, we can just say we are using the code in Firefox under the Apache license (that's the license most people are expected to use, out of the two) and that's already present. So no real need here. The sources will, of course, remain dual-licensed, as will our contributions." Gerv
Per comment #1.