You may need to remove the old password first.
Tools -> Options -> Security-> Passwords -> View Saved Passwords then selecting the password and then pressing the Remove button. (
http://kb.mozillazine.org/Setting_an...mail_passwords)
or
Click the TB Menu button (3 horiz.bars), then Options/Options/Security/Passwords/Saved Passwords, Show Passwords, right-click the account incoming and outgoing entries, Edit Password, enter the new password, OK to close Options, restart TB. (
https://support.mozilla.org/en-US/questions/1176322)