Maybe enable about:config in TBA stable

I know Firefox restricted this (for good reason, because making some changes can make the entire app/profile unrecoverable - losing data is not an issue for TB users, but it is a pain point). I'm preeeety sure we overrode this, yes? no? It makes testing/diagnosis harder when it's not enabled, although alpha is always available (but users diagnosing for us may not have it)

Feel free to close if this is not a regression, or we don't want to enable it.

ni @pierov