I've been using mail-in-a-box for 5 years and I couldn't be happier. For me, stability is the #1 concern for an email server, and mail-in-a-box is really set it and forget it. I also like that it includes CalDAV and CardDAV, so it served as a complete substitute to Google Mail+Contacts+Calendar.
I don't think that was due to Mozilla pushing Google. I have Firefox configured with DuckDuckGo on all my devices and this has never happened to me on an update.