Can I ask you to clarify what you're trying to do? Add the gmail account to Thunderbird? Or see some other account in Gmail? You don't have a "Thunderbird account" or "Mozilla account"; what you are using in Thunderbird will be offered by a mail provider. Folk who say "Thunderbird account" usually have one that is provided by their ISP as part of their internet access bundle. In either case, your first stop is probably the gmail settings web page. To add a gmail account to Thunderbird, you'll need to enable either POP or IMAP. To add a non-gmail account to gmail, you'll have to add it to gmail in the gmail settings web page. Try "Accounts and Import" then "Check mail from other accounts", or go to your email provider's webpage and see if it lets you forward your messages to your gmail account. And look at "Add another email address that you own" in the gmail site.