I recently installed jemalloc and was surprised that it did not cause SpiderMonkey to be rebuilt. Looking at the ebuild, it just adds --enable-jemalloc to the configure options, and it looks like the upstream version does not consider it an error for jemalloc to not be present when asked for.
(In reply to comment #0) > I recently installed jemalloc and was surprised that it did not cause > SpiderMonkey to be rebuilt. Installing a possible compile time dependency after installing wouldn't normally cause anything to be (automatically?) rebuilt.
--enable-jemalloc does not enable system jemalloc, no mozilla product uses system jemalloc.