have you added your gmail account to Thunderbird?

Add gmail account/email address to create a mail account in Thunderbird. You may need to have preferences set up to allow cookies, so do this first. Menu app icon > Preferences > Privacy & Security select checkbox 'Accept cookies from sites' Then create account: Menu app icon > New > Existing mail account Complete the details, enter name, email address and password, click on continue. Please note that the gmail account will automatically locate settings for an IMAP account. You will get a prompt by gmail requesting password to allow Thunderbird access. Please be aware that an IMAP account displays whatever is on server; you should not consider an imap account as a separate independent copy. If emails are fully downloaded then you can create a backup. If you do not want to use an IMAP - you want to set up as POP then you need click on 'Manual Config' button change the type to POP and alter the server settings to gmail's pop servre settings. Please note for POP access - gmail requires you to access webmail and select the 'less secure apps' option. Alternative is to set up/create an App password and use that app password instead of normal password in Thunderbird. https://support.google.com/accounts/answer/6010255