People are getting hung up on pedantic definitions of "hate speech". If we say "hostile work environment" instead, then the anonymous poster's behavior clearly crosses the line.
The response is not unique to Mozilla. Big companies like Google have probably encountered similar situations. Mozilla is just more open about it.
If you're tolerant of everything, including intolerance, you end up supporting intolerance.
Or, in short, it's perfectly fine to kick people out for being hateful. There's no inconsistency or hypocrisy in that position.