Google no longer allows a plain account password in mail apps for most accounts. Thunderbird should use OAuth2, a sign in window from Google.

1. Remove the account and add it again by entering only your address in Account Setup. Thunderbird detects Gmail and opens a Google sign in page. Sign in there and allow access.
2. If you added it manually, set the authentication method to OAuth2 for both incoming and outgoing servers.
3. Make sure IMAP is turned on in Gmail settings.
4. If your account uses 2 step verification and you really need a password based sign in, create an App password in your Google account, but OAuth2 is the better way.

Update Thunderbird as well, since older versions do not support the sign in properly.