Clone it from here: https://github.com/Aris-t2/CustomCSSforFx


Modify:
- UserContent.css:

@import "./css/aboutaddons/addons_manager_alternative_appearance_fx68.css"; /**/
@import "./css/aboutaddons/addonlists_show_addon_version_number_fx68.css"; /**/
@import "./css/aboutaddons/addonlists_private_browsing_notice_hidden.css"; /**/
@import "./css/aboutaddons/recentupdates_category_always_visible.css"; /**/
@import "./css/aboutpreferences/preferences_alternative_appearance.css"; /**/
@import "./css/aboutlogins/aboutlogins_alternative_appearance.css"; /**/
@import "./my_userContent.css"; /**/


- userChrome.css:

@import "./css/tabs/tabs_below_navigation_toolbar_fx65_v2_macOS.css"; /**/
@import "./my_userChrome.css"; /**/




Create file "my_userChrome.css" with:


#navigator-toolbox {
    position: relative;
}

.tabbrowser-tab {
    height: calc(var(--tab-min-height_tnot) + 1px);
}

@media (-moz-proton) {
    #TabsToolbar {
	--tab-min-height: calc(var(--tab-min-height_tnot) + 1px);
	--toolbarbutton-inner-padding: 4.5px !important;
    }

    .tab-content {
	height: 90%;
    }
}