src/third_party/mozjs-45/js/src/tests/js1_8_5/extensions/browser.js

// The page loaded in the browser is jsreftest.html, which is located in
// js/src/tests. That makes Worker script URLs resolve relative to the wrong
// directory. workerDir is the workaround.
workerDir = (document.location.href.replace(/\/[^/?]*(\?.*)?$/, '/') +
             'js1_8_5/extensions/');