My understanding is the only thing that uses the Lightning credential at Google these days is the provider for google calendar add-on. DO you have that installed? Perhaps ask the add-on author what the problem is.

if you had to enter your email address manually, it usually means cookies aren't accepted in TB Settings/Privacy & Security, and the OAuth authentication doesn't complete and store a token in Saved Passwords (which is read automatically at each connection). Accept cookies, restart TB, and see if the address appears automatically, and enter the password.

Thanks to both of you for your replies.

Hi Matt, I don't know if I have the Google calendar add-on. How do I find out if I have that? And If I do have it, how do I find out who the author is?

Hi S F Howes, My problem isn't related to entering my email address. The problem is that after I enter my password, it says it's the wrong password - but I know it's the right password. And this dialog box just started popping up a couple of days ago. Prior to that, I had never seen it. And I didn't make any changes to TB prior to the dialog box starting to pop up.

Thanks Clint