You can do a toolbar button but you can explain what exactly you like do with this button?
This is K-Meleon (not Firefox), not exist "Tools - Options - Safety - saved password", exist "Tools -> Privacy -> View Data -> View Passwords".
You can create a toolbar button with a
macro using in it
toolbar plugin commands and this function to open Password Viewer:
id(ID_PASSWORDS_VIEWER);
K-Meleon in Spanish