Sad to hear, Thunderbird has been my major and actually sole email client for a few years, not a Windows user so outlook is out, do I have to use mutt from now on? On Android I use K-9 which works well, it seems for Desktop the web email will be my only choice, not good at all.
I don't mind paying for Thunderbird, hope someone picks up the development. Firefox is great but there are alternatives, Thunderbird is great and _unique_ (across platform and an overall good product), I somehow feel Thuderbird is of more use to me than Firefox itself.