Mail-Day Counter extension for Thunderbird 68.2+
To install, download MailDay.xpi file. Then, open Thunderbird and click Preferences icon -> Addons -> Addons -> Preferences -> Install Add-on From File. For more detailed instruction see the manual.
Adds a button to Thunderbird, which shows pop-up with total number of Mail-Days and information about the oldest e-mail. Concept of Mail-Days was described by Maxim Dorofeev (Russian language). In essense, total number of mail days is equal to sum of squares of number of days since each message was recieved over all unread messages.