This is fallout from bug 1336213. That added a field DOMParser::mOriginalPrincipleWasSystem but only initialised it in one of the two constructors for the class. Use of the uninitialised field is easily observed with Valgrind, from example on dom/xslt/tests/mochitest/test_bug427060.html.
Created attachment 8835528 [details] [diff] [review] bug1338156-1.cset The obvious fix.
Thanks for the quick fix!
Pushed by email@example.com: https://hg.mozilla.org/integration/mozilla-inbound/rev/d22eeada4c35 Add missing initialisation of DOMParser::mOriginalPrincipleWasSystem. r=ehsan.