While I too would love to see continued development of Persona and Thunderbird by Mozilla, it's important to remember that as open-source projects they're still freely available. I still use both, and see no reason to stop. Development can continue without Mozilla.
I used it two weeks ago for a weekend project (shameless plug: https://www.pastery.net/, a beautiful pastebin), and it's just not the same. Feature requests go into the void, although the team at Mozilla was kind enough to answer even though it's not their job any more, but you can't really use it for a "serious" site while it's very unsupported.
