Thunderbird - It's an albatros.
It's as old as the browser side of Mozilla so if it was going to be a success it would have followed in the wake of Firefox.
Spinning it off as a lone entity is just a measure to get rid of unwanted baggage. Your choice of web browser has become fashionable/religious whereas no one cares about email and still think only geeks bang on about using alternatives.
Are there any alternative email clients for Windows that are a success?