I can't reset Thunderbird password.

If it is a primary password (the error message would state if it's that one), only you can do that. Let us know. If it is an email account password, you may need to contact email provider. OR, give more information on what you mean by 'can't reset password.'

It's an email account.

If you need a new password, you must go to email provider's site. If it's a valid email password, then the account settings may be wrong. I still do not understand your specific situation. That is, since you have a password, why do you need to reset it? If you want to remove existing password, click settings>privacy&security and then click 'saved passwords' and then 'show passwords' - and then highlight and right-click to edit, where you can delete it, or highllight rows for the account and delete both. Then thunderbird will have no password and you can reset it.