I too have a problem signing into Gmail using Thunderbird. It refuses my password whilst I have no problem logging into Gmail using Chrome with the same password. Maybe I am a cynic but it could be that Google is trying to control which software can access Gmail as it is losing out on advertising.
I prefer to use Thunderbird as it is far more useable and versatile. There are no adverts whereas using Gmail in Chrome I get many unsolicited advertisements.
I have tried all the recommended solutions but none of them work.
I'm going to give it 30 days to see if things change after which I will use a new email account and stop using Gmail and Chrome altogether - ADVERTISERS IN GMAIL NOTE!