userChrome.css

/* Tighten up drop-down/context/popup menu spacing. */
menupopup > menuitem, menupopup > menu {
  padding-block: 3px !important;
}
:root {
  --arrowpanel-menuitem-padding: 4px 8px !important;
}

/* Square tabs. */
#tabbrowser-tabs {
    --tab-border-radius: 0px;
}

/* Scrollback buttons. */
#scrollbutton-up, #scrollbutton-down {
    --tab-border-radius: 2px;
    border: 1px solid transparent !important;
    min-width: 26px !important;
    margin-right: 1px !important;
}
#scrollbutton-down {
    margin-left: 2px !important;
}

/* Fix styling on the new tab button variant that exists when there is fewer tabs with no scroll buttons. */
#tabs-newtab-button {
    --tab-border-radius: 4px;
}

/* Remove space below tabs. */
.tab-background {
    border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important;
    margin-block: 1px 0 !important;
}

/* Adjust the border on the selected tab, removing the outline and adding a border. */
.tab-background[selected] {
    outline: none !important;
    border: 2px solid var(--lwt-tab-line-color, var(--lwt-tabs-border-color, currentColor)) !important;
    border-bottom: none !important;
}

/* Move the container indicator down below the border outline. */
.tab-context-line {
    margin: 0px 2px 0 !important;
}

/* Inactive tabs: Separator lines. */
.tabbrowser-tab:not([beforeselected-visible="true"]) .tab-background:not([selected]):not([multiselected]) {
    border-right: 1px solid rgba(0, 0, 0, .20) !important;
}
[brighttext="true"] .tabbrowser-tab:not([beforeselected-visible="true"]) .tab-background:not([selected]):not([multiselected]) {
    border-right: 1px solid var(--lwt-selected-tab-background-color, rgba(255, 255, 255, .20)) !important;
}
.tab-background:not([selected]):not([multiselected]) {
    border-radius: 0 !important;
}

/* Remove padding between tabs. */
.tabbrowser-tab {
    padding-left: 0 !important;
    padding-right: 0 !important;
}

/* Fixes the text color when highlighting text in an input box. */
/* Sets it to white. */
input::selection {
/*    color: rgba(255, 255, 255, 1) !important;*/
    color: var(--in-content-item-selected-text, rgb(251,251,254)) !important;
    background-color: var(--in-content-item-selected, #0061e0) !important;
}

/* Turns menus back into light themed menus. */
/* These values are taken directly from global.css:257 for Windows 10. */
menupopup {
    --menuitem-hover-background-color: #e0e0e6 !important;
    --menu-background-color: #f9f9fb !important;
    --menu-color: #15141a !important;
    --menuitem-disabled-hover-background-color: rgba(224, 224, 230, 0.4) !important;
    --menu-disabled-color: rgba(21, 20, 26, 0.4) !important;
    --menu-border-color: #cfcfd8 !important;
    --menu-icon-opacity: 0.7 !important;
}

/* Enlarge the tabs very slightly. */
:root[uidensity="compact"] {
    --tab-min-height: 27px !important;
}
#tabbrowser-tabs:not([secondarytext-unsupported]) .tab-label-container {
    height: 2.5em !important;
}