Safari runs on many, many millions of various Apple devices though, and I think that more than qualifies it as a major/mainstream browser, and the same for Webk

Safari runs on many, many millions of various Apple devices though, and I think that more than qualifies it as a major/mainstream browser, and the same for Webkit as a mainstream engine... especially considering Safari's market share is ~5x higher than Firefox. You may disagree though, and that's ok.