My Firefox userChrome.css file

userChrome.css

/* Fira Code font for devtools */
.CodeMirror {
    font-family: "Fira Code", monospace !important;
    font-size: 18pt !important;
}

/* slimmer and more minimal header */
#urlbar {
    --urlbar-toolbar-height: 32px !important;
}

#urlbar-container {
    --urlbar-container-height: 32px !important;
}

#urlbar-input-container {
    --urlbar-icon-fill-opacity: 0.75;
}

#urlbar-background {
    --toolbar-field-background-color: rgba(53, 54, 61, 0.5) !important;
}

.tabbrowser-tab {
    padding: 0 0 !important;
}

.titlebar-button {
    padding: 4px 12px !important;
}

#TabsToolbar #firefox-view-button[open] > .toolbarbutton-icon,
#tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
    box-shadow: 0 0 0px rgba(0, 0, 0, 0.4) !important;
}

:root:-moz-lwtheme {
    --focus-outline-color: #989898 !important;
}

:root {
    --toolbar-start-end-padding: 0px !important;
    --toolbarbutton-outer-padding: 0px !important;
    --toolbarbutton-border-radius: 0px !important;
    --urlbar-margin-inline: 0px !important;
    --focus-outline-width: 1px !important;

    --tab-min-height: 32px !important;
    --tab-border-radius: 0px !important;
    --tab-block-margin: 2px !important;

    --arrowpanel-border-radius: 2px !important;
    --arrowpanel-menuitem-border-radius: 2px;
}