You may have to enable less secure apps in your Google account settings. https://support.google.com/accounts/answer/6009563?hl=en

DocVogue said

I am consistently getting "Login to server imap.googlemail.com failed.", on one Gmail account even though settings are identical to other gmail accounts that work perfectly. Would be grateful for a solution.

As I thought I was in a Thunderbird forum for Mozilla, I wasn't as specific as I needed to be. The "Login to server imap.googlemail.com failed." error comes up when using Thunderbird and trying to access a Gmail account. There are no "apps" involved.

I'd suggest you make the effort and read the Google support article linked above.

Hmm, as far as Gmail is concerned, Thunderbird is a "less secure app(lication)" and requires the user to specifically permit it to be used.

It appears to be Google's choice to call everything an "app". https://support.google.com/accounts/answer/6010255?hl=en

Indeed you need to read a tad more carefully.

I see devs talking about OAuth2, which when introduced should make this option redundant.