Well This is not possible. Minimize, Maximize, and Close button theme is defined by system or in your case Windows . So if you want to change that theme to match with Firefox you will need to change the windows theme with a matching one. Using the default windows theme is recommended for proper match because theme designers create their themes by taking it as standard. so it should match with every Firefox theme. Using the default Minimize, Maximize, and Close button windows theme will perfectly match with your browser's theme or change your browsers theme to match with system theme. either way your choice. Thank you.