Thunderbird is the only MOZ product that I still use daily - almost at par with Mail.app if not more, and I hope to keep using it unless they eventually release the iOS Thunderbird after making it unrecognisable to me and ensuring that some of the differentiating Thunderbird features are missing – like the ability to send email from any address on a domain by just editing the "From" field - of course, it will work only if you own that domain. But it's a feature I can't do without (and utilise it a lot on desktop). Then there are forever pending things like maildir support :)