Tweak FirefoxBinary to allow easier subclassing

As explained at http://pad.lv/1161336 , I have specific use case where I do *not* want to use MOZ_NO_REMOTE so I can trap apps calling external urls (I don't control those apps).

This patch moves the setting of the env vars right ater the firefox_env is created instead of _start_from_profile_path (which is private as denoted by the leading _).

Doing so allows one to derive from FirefoxBinary which may also address other needs.

It will still be up to subclasser to provide the new class, this patch doesn't change the actual behaviour.