There are at least two issues: 1) we try to copy the plugins folder (see bug 1057328) which doesn't exist 2) we try to use certutil.exe for something, which also doesn't exist (not in the firefox directory passed through appname, or in our own $OBJDIR/dist directory). I've not gotten further than that. Ideally, I'd actually like the tests to work without even an $OBJDIR, as it should in theory all just be some python... but I guess in the absence of that, it'd be great if it worked with --disable-compile-environment, at least. Note that the --disable-compile-environment currently needs a few fixes/hacks to work (see bug 1160798).
Turns out that there are a bunch of build related binaries that the mochitest harness uses: https://dxr.mozilla.org/mozilla-central/source/testing/mochitest/Makefile.in#18 The good news is that these get packaged in the tests.zip, and with the patch in bug 1171602 applied, you can run |mach mochitest --appname <path to some other binary>| directly from a tests.zip. I'm not sure if running from a tests.zip satisfies your requirements or not, but I aim to have it landed by the end of the quarter. Getting mochitest to run without either an objdir or a tests.zip would be a massive undertaking.
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