After adding two loadFrameScript calls to Talos, (at http://hg.mozilla.org/build/talos/annotate/8ef116144662/talos/pageloader/chrome/pageloader.js#l213 ), our Talos xperf main thread IO test complains that these files are being accessed on the main thread. This caused us to revert to a previous Talos revision in bug 1123852. I don't have call stacks for these file accesses.
nsFrameScriptExecutor::TryCacheLoadAndCompileScript We do cache the scripts so after startup the scripts are just cloned to new tabs etc.
...and that setup is quite similar to mozJSSubScriptLoader.cpp