Firefox 89 - userChrome.css

#main-window .tab-background[multiselected="true"],
#main-window .tab-background[selected="true"]{
    /* Make active tab background match address bar color */
    background-color: var(--toolbar-bgcolor) !important;
    background-image: var(--toolbar-bgimage) !important;
    color: var(--toolbar-color) !important;
}
#main-window:not([lwtheme="true"]) .tab-background[multiselected="true"],
#main-window:not([lwtheme="true"]) .tab-background[selected="true"]{
    /* Used with system theme - May not look good on every system theme */
    border: 1px solid var(--tabs-border-color) !important;
    border-bottom: none !important;
}

.tab-background {
    /* Make tabs not look like buttons */
    margin: 1px 0 0 !important;
    --tab-border-radius: 8px 8px 0 0 !important;
}
toolbarbutton#scrollbutton-up[part="scrollbutton-up"],
toolbarbutton#scrollbutton-down[part="scrollbutton-down"] {
    /* Prevent needing to alter tab height
     * This is for the tab scrolling buttons
     * The change to the tab-brackground margin
     * causes the border size a issue here */
    border: none !important;
}

:root[uidensity="compact"] .tab-close-button{
    /* Adjust size of close tab button in compact mode */
    padding: 3px !important;
    margin: 0 !important;
    height: 20px !important;
    width: 20px !important;
}

#tabbrowser-tabs .tabbrowser-tab:not([pinned]){
    /* Tab minimum width */
    min-width: 110px !important;
}
#tabbrowser-tabs[closebuttons="activetab"] .tabbrowser-tab:not([selected="true"]):not([pinned]):hover .close-icon{
    /* Show close button even when tabs are low on width */
    display: -moz-box !important;
}

#tabbrowser-tabs .tabbrowser-tab[pending]:not([selected="true"]):not(:hover){
    /* Distinguish between loaded and unloaded tabs */
    opacity: 0.5;
}

#toolbar-menubar > spacer{
    /* Remove spacer in menu bar */
    display: none;
}
#toolbar-menubar toolbarspring{
    /* Make flexable spaces max length in menu bar */
    max-width: none !important;
}