in about:config, enable toolkit.legacyUserProfileCustomizations.stylesheets

$ cat .mozilla/firefox/<PROFILE>/chrome/userChrome.css

#TabsToolbar {
    visibility: collapse !important;
}

#main-window[tabsintitlebar]:not([sizemode="fullscreen"]) #TabsToolbar:not([collapsed="true"]) + #nav-bar:not(:-moz-lwtheme) {
    background-clip: padding-box;
    position: relative;
    z-index: 1;
    padding-left: 67px;
}