You have removed the emailaccount via the accountsettings in thunderbird? and when you open thundebird it still ask to login in google? first of all try to remove the cookies via settings > privacy and security

My best guess is there is still an address book linked to Google Contacts, or a Google Calendar, or both. Check your settings.

Thank you both for your responses! I've removed the cookies, and I've looked over all of my settings and can't find a trace of google anywhere. Is there something I'm missing?

Should I just try to uninstall thunderbird and re-do everything?

I've looked over all of my settings and can't find a trace of google anywhere.

Not sure what that means. You need to check for a Google Calendar and/or an address book linked to Google Contacts.

Should I just try to uninstall thunderbird and re-do everything?

Re-installing Thunderbird will keep your Thunderbird profile, so you'd just be wasting your time. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data

if you go to Help => troubleshootinformation do you see somewhere on that page a mention to your old google account?