Hi,
I can't comment on Thunderbird or Squirrel Mail, but recently I have been hearing a lot about
Mailpile. AFAIK, it's under heavy development.
Here is what the website says:
Mailpile - A modern, fast web-mail client with user-friendly encryption and privacy features. 100% Free and Open Source software
Privacy
Store your mail on devices you control, encrypt it and share or restrict access as you see fit.
You can even encrypt the local settings and search index so if your computer gets stolen, your mail stays secure.
Encryption
OpenPGP signatures and encryption are part of Mailpile's core design, not an afterthought or plugin.
An intuitive, modern user interface makes strong security accessible to everyone.