My favourite thunderbird extensions

To check all IMAP folders for new messages (no need to click on a folder to see if there's unread messages there):
Edit -> Preferences -> Advanced -> General -> Config Editor: mail.check_all_imap_folders_for_new = true

Display IMAP quota:
Edit -> Preferences -> Advanced -> General -> Config Editor: mail.quota.mainwindow_threshold.show = 0

Disable "flowed" formatting:
Edit -> Preferences -> Advanced -> General -> Config Editor: mailnews.send_plaintext_flowed = false

Show more recipients, etc. in headers:
Edit -> Preferences -> Advanced -> General -> Config Editor: mailnews.headers.show_n_lines_before_more = 3

Don't strip signature (or any other text below the "-- " line, such as a quoted e-mail) when replying:
Edit -> Preferences -> Advanced -> General -> Config Editor: mail.strip_sig_on_reply = false

Disable annoying automatic selection of messages:
Edit -> Preferences -> Advanced -> General -> Config Editor: mailnews.remember_selected_message = false

To avoid getting very long lines when reading e-mails in wide windows, add this to ~/.thunderbird/*/chrome/userContent.css (based on this page):

.moz-text-plain {
  max-width: 120ch !important
}
.moz-text-flowed {
  max-width: 80ch !important
}
.moz-text-html {
  max-width: 60em !important
}

See also: My favourite firefox extensions

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