CMEX-feel пишет

может подойдет:

скрытый текст

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

#TabsToolbar .tabbrowser-arrowscrollbox,
#TabsToolbar .arrowscrollbox-scrollbox,
#TabsToolbar .scrollbox-innerbox {
display: block !important;
}
.scrollbutton-up,
.arrowscrollbox-overflow-start-indicator,
.scrollbutton-down,
.arrowscrollbox-overflow-end-indicator {
display: none !important;
}
.tabbrowser-tab {
min-height: var(--tab-min-height) !important;
vertical-align: bottom !important;
}
/* Изменить высоту вкладок */
/* -------------------------- */
tab {
height: 10px !important;
}
.tabbrowser-strip {
height: 12px !important;
}

/*
* Edit this file and copy it as userChrome.css into your
* profile-directory/chrome/
*/

/*
* This file can be used to customize the look of Mozilla's user interface
* You should consider using !important on rules which you want to
* override default settings.
*/

/*
* Do not remove the @namespace line -- it's required for correct functioning
*/

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

/*
* For more examples see http://www.mozilla.org/unix/customizing.html
*/

@-moz-document url("chrome://browser/content/browser.xul") {
#main-window:not([inFullscreen]) #navigator-toolbox {
    position: relative !important;
    --v-toolbar-button-inner-padding: var(--toolbarbutton-inner-padding, 6px);
}
#main-window:not([inFullscreen]) #navigator-toolbox > toolbar {
    -moz-box-ordinal-group: 10 !important;
}
#main-window:not([inFullscreen]) #navigator-toolbox > #nav-bar {
    margin-top: calc(2 * var(--v-toolbar-button-inner-padding) + 16px) !important;
    margin-bottom: 0 !important;
    box-shadow: none !important;
    -moz-box-ordinal-group: 0 !important;
    padding-left: 0 !important;
    padding-right: 0 !important;
}
#main-window:not([inFullscreen]) #navigator-toolbox > #PersonalToolbar {
    -moz-box-ordinal-group: 1 !important;
}
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="normal"] > #navigator-toolbox {
    -moz-appearance: -moz-window-titlebar !important;
}
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="maximized"] > #navigator-toolbox {
    -moz-appearance: -moz-window-titlebar-maximized !important;
}
#main-window:not([inFullscreen])[tabsintitlebar]:-moz-lwtheme > #navigator-toolbox {
    visibility: hidden !important;
}
#main-window:not([inFullscreen])[tabsintitlebar]:-moz-lwtheme > #navigator-toolbox > *:not([collapsed="true"]) {
    visibility: visible;
}
#navigator-toolbox > #titlebar {
    -moz-appearance: none !important;
    -moz-box-ordinal-group: 100 !important;
    --v-control-buttons-left: auto;
    --v-control-buttons-right: 0;
    position: static !important;
}
#navigator-toolbox > #titlebar:-moz-locale-dir(rtl) {
    --v-control-buttons-left: 0;
    --v-control-buttons-right: auto;
}
#main-window:not([inFullscreen]) #navigator-toolbox::after,
#main-window:not([inFullscreen]) #TabsToolbar::after {
    display: none !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar {
    padding-top: 0 !important;
    padding-bottom: 0 !important;
    margin-top: 0 !important;
    margin-bottom: 0 !important;
    position: absolute !important;
    top: 0 !important;
    left: 0 !important;
    right: 0 !important;
    display: flex !important;
    flex-wrap: nowrap !important;
    flex-direction: row !important;
    align-items: stretch !important;
    justify-content: flex-start !important;
    border: none !important;
    background: none !important;
    box-sizing: content-box !important;
    min-height: 0 !important;
    height: calc(2 * var(--v-toolbar-button-inner-padding) + 16px) !important;
    overflow: hidden !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar > :-moz-any(toolbaritem,toolbarbutton) {
    align-self: center !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar > :-moz-any(#menubar-items,#wrapper-menubar-items,.titlebar-buttonbox-container) {
    align-self: flex-start !important;
}
#main-window[inFullscreen] #toolbar-menubar {
    visibility: collapse !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar > * {
    padding-top: 0 !important;
    padding-bottom: 0 !important;
    margin-top: 0 !important;
    margin-bottom: 0 !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar[autohide="true"][inactive="true"]:not([customizing="true"]) > * {
    opacity: 0 !important;
    pointer-events: none !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar > * > * {
    vertical-align: middle;
}
#main-window:not([inFullscreen]) #toolbar-menubar > #menubar-items > * {
    vertical-align: top;
}
#main-window:not([inFullscreen])[tabsintitlebar] #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
    position: absolute !important;
    top: 0px !important;
    left: var(--v-control-buttons-left) !important;
    right: var(--v-control-buttons-right) !important;
    margin: 0 !important;
    padding: 0 !important;
    font-size: 0px !important;
}
@media (-moz-windows-classic) {
#main-window:not([inFullscreen])[tabsintitlebar] #toolbar-menubar,
#main-window:not([inFullscreen])[tabsintitlebar] #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
    padding-top: 4px !important;
}
#main-window:not([inFullscreen])[tabsintitlebar] #toolbar-menubar[autohide="true"] {
    min-height: 4px !important;
}
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="normal"]:-moz-lwtheme #toolbar-menubar {
    background-image: linear-gradient(to bottom, ThreeDLightShadow 0, ThreeDLightShadow 1px, ThreeDHighlight 1px, ThreeDHighlight 2px, ActiveBorder 2px, ActiveBorder 4px, transparent 4px) !important;
}
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="normal"] #nav-bar {
    margin-top: calc(2 * var(--v-toolbar-button-inner-padding) + 20px) !important;
}
}
@media (-moz-windows-compositor), (-moz-os-version: windows-win7) and (-moz-windows-default-theme) {
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="maximized"] #toolbar-menubar,
#main-window:not([inFullscreen])[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
    padding-top: 8px !important;
}
}
#main-window:not([inFullscreen]) #toolbar-menubar > *[ordinal="1000"],
#main-window:not([inFullscreen]) #toolbar-menubar > .titlebar-buttonbox-container {
    order: 1000 !important;
}
#main-window:not([inFullscreen]) #toolbar-menubar > :-moz-any(toolbarspring,spacer,[id^="wrapper-customizableui-special-spring"]) {
    flex-grow: 1 !important;
}
#main-window:not([inFullscreen]) #TabsToolbar {
    margin-top: 0 !important;
    margin-bottom: 0 !important;
    padding-top: 0 !important;
    padding-bottom: 0 !important;
    box-shadow: 0 -1px 0 var(--tabs-border-color, rgba(0,0,0,.3)) inset !important;
    position: static !important;
}
#main-window:not([inFullscreen]) #TabsToolbar #tabbrowser-tabs {
    padding-bottom: 0 !important;
    margin-bottom: 0 !important;
}
#main-window:not([inFullscreen]) #TabsToolbar > .toolbar-items {
    padding-top: 0 !important;
    margin-top: 0 !important;
}
#main-window:not([inFullscreen]) #TabsToolbar .tab-background {
    border-top-style: solid !important;
}
#main-window:not([inFullscreen]) #TabsToolbar .titlebar-spacer:-moz-any([type="pre-tabs"],[type="post-tabs"]) {
    display: none !important;
}
}

/* Firefox Quantum userChrome.css tweaks ************************************************/
/* Github: https://github.com/aris-t2/customcssforfx ************************************/
/****************************************************************************************/

/* tab close - always visible*/
#TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned]) .tab-close-button {
  visibility: visible !important;
  display: block !important;
}
#TabsToolbar #tabbrowser-tabs .tabbrowser-tab:not([pinned])[faviconized="true"] .tab-close-button {
  visibility: collapse !important;
  display: none !important;
}

/**/

Meraiya11 пишет

огромное спасибо

не за что.... мне тоже подкинули... ))   а высоту вкладок как менять?

Просто код, он какой-то невеселый...