If you have changed your user password that you use email services on you'll need to update your mail clients too. To update your mailbox password in Mozilla Thunderbird simply start the application. It will notice that the password no longer works and prompt you for the new one.
Enter it in the window that appears and select the Use Password Manager to remember this password box before clicking OK.
If for any reason the above instructions do not work, you can delete the saved password and start again. To do this, select Tools from the top menu, then Options.
Select Security in the window that opens and then the Passwords.
Select the Saved Passwords button and find your account from the list and then the Remove button. Exit Thunderbird and restart - you should be prompted for your password and now have no issues.