I'm having the exact same problem as described in this question: https://support.mozilla.org/en-US/questions/1440106 I had a university email account in Thunderbird, and then I attempted to delete the email from Thunderbird as it had been deactivated. I deleted its entries from saved passwords / the calendar. However, I'm still getting a googleapis.com login prompt every time I open Thunderbird, although I can find no references for it anywhere. The answer to the linked question recommends checking the address book for a caldav link to the old account, but I don't see anything like that so either I'm looking in the wrong place or that's not my problem. I appreciate the help, I love Thunderbird but this is getting really annoying.