According to (here)[https://code.google.com/p/rangy/issues/detail?id=6], Rangy needs to be initialized itself before we can use it.

However, it appears that on firefox, mocha starts running before Rangy has a chance to initialize itself, here we need to ensure rangy is initialized before using
Fix backwards tests on firefox
Fix DOM-related tests on firefox
Fix exceptions
Merge branch 'xxuejie-fix-firefox-tests'