There is now a global jit USE flag with the following description: "Enable just-in-time compilation for improved performance. May prevent use of some PaX memory protection features in Gentoo Hardened." If the package's local methodjit USE flag does not offer valuable information beyond the global flag's description, consider switching to the global flag and removing the local flag from metadata.xml.
methodjit and tracejit are not compilers.
(In reply to comment #1) > methodjit and tracejit are not compilers. With all due respect, I did not understand your comment at all. If you are referring to the traditional distinction between compilers and interpreters, the acronym JIT (as e.g. used in "methodjit") has always stood for "just-in-time compilation" (see http://en.wikipedia.org/wiki/Just-in-time_compilation or your favorite dictionary of computing). You may of course argue that "just-in-time compilation" is not really compilation at all, but that's about as pointless as refusing to use the phrase "mountain lion" because it's not really a lion, or "soy milk" because it contains no milk. Noun phrases in the English language very frequently mean something other than the literal conjunction of their component words. If on the other hand you meant that Mozilla does not internally use the word "compiler" when referring to methodjit, then you are simply mistaken: "compiler" occurs 9 times on https://wiki.mozilla.org/JaegerMonkey
