For FirefoxGX (Not tested in normal firefox). Adding the macos-inspired min/max/close buttons.

macos_like_buttons_for_windows_firefoxgx.css

/* macOS-inspired  Window Buttons */
/* 
Usage:
1. Place this file in your Firefox profile's "chrome/components" directory.
2. Import this file in your userChrome.css:
   @import url('components/macos_windows_buttons.css');
3. Disable any other window button styles to avoid conflicts (In this case, comment out ogx_windows-controls.css in userChrome.css).
*/

/* 1. Position buttons on the left */
#TabsToolbar.titlebar-buttonbox-container {
    order: -1 !important;
    margin-right: 10px !important;
    margin-left: 5px !important;
}

/* 2. Base button styles */
.titlebar-button {
    appearance: none !important;
    -moz-appearance: none !important;
    padding: 0 !important;
    margin: 0 4px !important;
    margin-top: 6px !important;
    width: 16px !important;
    height: 16px !important;
    min-width: 16px !important;
    min-height: 16px !important;
    border-radius: 50% !important;
    background-position: center !important;
    background-repeat: no-repeat !important;
    transition: opacity 0.2s ease !important;
}

/* 3. Traffic light colors */
.titlebar-close {
    background-color: #ff5f57 !important;
    background-image: radial-gradient(#ff5f57 55%, transparent 65%) !important;
    color: rgba(0, 0, 0, 0.9) !important;
    border: 1.5px solid #E04448 !important;
}

.titlebar-min {
    background-color: #28c840 !important;
    background-image: radial-gradient(#28c840 55%, transparent 65%) !important;
    color: rgba(0, 0, 0, 0.9) !important;
    border: 1.5px solid #1FAA40 !important;
}

.titlebar-max,
.titlebar-restore {
    background-color: #ffbd2e !important;
    background-image: radial-gradient(#ffbd2e 55%, transparent 65%) !important;
    color: rgba(0, 0, 0, 0.9) !important;
    border: 1.5px solid #DE9D41 !important;
}

/* 4. Hover states */
.titlebar-close:hover {
    background-color: #BF4744 !important;
    background-image: radial-gradient(#BF4744 55%, transparent 65%) !important;
}

.titlebar-min:hover {
    background-color: #279637 !important;
    background-image: radial-gradient(#279637 55%, transparent 65%) !important;
}

.titlebar-max:hover,
.titlebar-restore:hover {
    background-color: #BD8C2E !important;
    background-image: radial-gradient(#BD8C2E 55%, transparent 65%) !important;
}

/* 5. Maximized window adjustment */
#main-window[sizemode="maximized"] .titlebar-button {
    margin-top: 8px !important;
}