Have you removed the stored old password from Thunderbird? In gmail webmail have you selected to use what gmail term as 'less secure apps' option? If yes, at the prompt to enter password, did you also select to remember the password? If yes, is the correct password now stored in Thunderbird?

Where are passwords stored? Tools > Options > Security > Passwords tab or Menu icon > Options > Options > Security > Passwords tab click on 'Saved Passwords' click on 'show Passwords'