For people who like dark themes, this works in 99.999% of the cases. You need to copy this code, save it as a userContent.css file. Move it to the chrome folder inside the default .Mozilla profile folder. If there is no chrome folder, just create one. The other thing you can do of course is find a theme or persona you like that matches your theme.
input {
border: 2px inset white;
background-color: white;
color: black;
-moz-appearance: none !important;
}
textarea {
border: 2px inset white;
background-color: white;
color: black;
-moz-appearance: none !important;
}
select {
border: 2px inset white;
background-color: white;
color: black;
-moz-appearance: none !important;
}
input[type="radio"],
input[type="checkbox"] {
border: 2px inset white ! important;
background-color: white ! important;
color: ThreeDFace ! important;
-moz-appearance: none !important;
}
*|*::-moz-radio {
background-color: white;
-moz-appearance: none !important;
}
button,
input[type="reset"],
input[type="button"],
input[type="submit"] {
border: 2px outset white;
background-color: #eeeeee;
color: black;
-moz-appearance: none !important;
}
body {
background-color: white;
color: black;
display: block;
margin: 8px;
-moz-appearance: none !important;
}
edit: path to the folder /home/(user)/.mozilla/firefox/"a slew of letters".default/chrome