My guess is you want a password to prevent starting Thunderbird. That is called the 'primary password' and can be set at tools>settings>privacy&security under PASSWORDS. Be sure to remember it, because there is no easy way to recover if you forget it.

Thank you David.

No, I wanted Thunderbird to download e-mails from my (two) ISP servers. Previously, Comcast was downloading from my other e-mail server and sending all e-mails to my PC, but after Thunderbird (including my address-book!) spontaneously disappeared from my PC and after multiple attempts to re-install Thunderbird and then rebuilding my address-book, I was randomly having to manually re-enter the password for my primary ISP.

The solution was to add a secondary e-mail ISP log-in (comcast), then make it my primary. BINGO! (so far). Apparently, when I re-installed Thunderbird, after its mysterious vanishing act, the previous settings were corrupted in some way. At least I still have my many-years collection of e-mails, from which I could re-construct my address-book!