I recently noticed Thunderbird now supports the Gmail online authentication window that appears, and lets you sign in to Google directly, instead of having to make an app-specific password. That's great. However, there's a bug. It seems to keep popping up this window over and over, and it keeps asking for the email address. No matter what is entered, the window doesn't advance to the next stage (asking for password, sending the authentication code to your phone, and so on). It just stays locked on the email address page. I think I know what's going on. Thunderbird has its own internal Web browser, that is used for situations like this, rendering the authentication window. That browser has its own cookie settings. Here's what I found that worked for me: Tools / Preferences / Privacy & Security / Accept cookies from sites / Exceptions In the Exceptions window, enter this site: https://accounts.google.com Then, hit Allow, then hit Save Changes. Keep the "Accept cookies from sites" box unchecked, there's no need to check it. After that, it worked! The Gmail authentication process continued normally. I was able to complete the Google authentication wizard for each of my Gmail accounts. Then, Thunderbird worked normally. This is a subtle thing, I wish that Thunderbird would detect the need for authentication to work, and grant a cookie exception automatically for the email accounts that need it.