tabs_kde.css

/* @import "./tabs_full_width.css"; */

@-moz-document url(chrome://browser/content/browser.xul)
{
    /* Your style rules for the user interface go here */

    :root
    {
        --custom-tab-border-roundness: 2px;
        --custom-tab-border-width: 1px;
        --custom-tab-spacing: 0px;
        --custom-toolbar-bgcolor: #eff0f1;
        --custom-toolbar-border-color: #afb1b3;
        --custom-tab-inactive-color: #cdcfd0;
        --custom-tab-close-button-size: 18px;
    }

    .tabbrowser-tab .tab-close-button
    {
        width: var(--custom-tab-close-button-size);
        height: var(--custom-tab-close-button-size);
    }

    .tabbrowser-tab .tab-loading-burst
    {
        display: none !important;
    }

    /* Enable the tab-line, to show multiple tab selection
    .tab-line
    {
        display: none !important;
    }
    */

    #TabsToolbar
    {
        background-image: unset !important;
        background-color: var(--custom-toolbar-bgcolor) !important;
    }

    .tabbrowser-tab:first-child
    {
        padding-left: 5px !important;
    }

    .tabbrowser-tab
    {
        padding-top: 5px !important;

        margin-left: var(--custom-tab-spacing) !important;
        margin-right: 0px !important;
    }

    .tabbrowser-tab .tab-background
    {
        border-top: var(--custom-tab-border-width) solid var(--custom-toolbar-border-color) !important;
        border-left: var(--custom-tab-border-width) solid var(--custom-toolbar-border-color) !important;
        border-right: var(--custom-tab-border-width) solid var(--custom-toolbar-border-color) !important;

        background-color: var(--custom-toolbar-bgcolor) !important;
    }

    .tabbrowser-tab .tab-background:not([selected="true"])
    {
        background-color: var(--custom-tab-inactive-color) !important;
    }

    /* tab top border roundness */
    #TabsToolbar .tabs-newtab-button,
    #TabsToolbar .tabbrowser-tab,
    #TabsToolbar .tabbrowser-tab .tab-stack,
    #TabsToolbar .tabbrowser-tab .tab-background,
    #TabsToolbar .tabbrowser-tab .tab-content
    {
        border-top-left-radius: var(--custom-tab-border-roundness) !important;
        border-top-right-radius: var(--custom-tab-border-roundness) !important;
        overflow: hidden;
    }

    /* hide tab separators */
    .tabbrowser-tab::after,
    .tabbrowser-tab::before
    {
        opacity: 0 !important;
        border-image: unset !important;
        border-image-slice: unset !important;
        width: unset !important;
    }
}

userChrome.css

@import "./tabs_kde.css";

@-moz-document url(chrome://browser/content/browser.xul)
{
    /* Your style rules for the user interface go here */
}