fenix-fox/chrome/combined_tabs_and_main_toolbars.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/combined_tabs_and_main_toolbars.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Combines tabs toolbar and nav-bar in such a way that opening new tabs will reserve
   space from nav-bar. Only pinned tabs and the selected tab will be visible, all other tabs need to be accessed via alltabs-button or shortcut keys */

/* You WILL NEED either autohide_menubar.css or overlay_menubar.css with this or use custom css that does something similar */  

#navigator-toolbox{
  --uc-urlbar-min-width: 50vw; /* minimum width for opened urlbar */
}

#toolbar-menubar[autohide="true"] > .titlebar-buttonbox-container{ display: none !important; }
#toolbar-menubar{ left:0 }

@supports -moz-bool-pref("layout.css.osx-font-smoothing.enabled"){
  #TabsToolbar > .titlebar-buttonbox-container{
    position: fixed;
    display: block;
    left: 0px;
    z-index: 1;
  }
  :root[tabsintitlebar="true"] #nav-bar{ padding-inline: calc(var(--uc-hide-window-control-space,1) * 96px + 20px) 0px !important; }
  
  @media (-moz-platform: windows-win10),(-moz-os-version: windows-win10){
    :root[tabsintitlebar="true"] #nav-bar{ padding-left: calc(var(--uc-hide-window-control-space,1) * 134px + 20px) !important; }
  }
  #toolbar-menubar[autohide="true"] + #TabsToolbar .titlebar-buttonbox{ -moz-box-direction: reverse }
}

#navigator-toolbox{ --tab-min-height: 40px }
:root[uidensity="compact"] #navigator-toolbox{ --tab-min-height: 32px }

#navigator-toolbox{
  display: flex;
  flex-direction: row-reverse;
  flex-wrap: wrap;
}

#titlebar{
  -moz-appearance: none !important;
  flex-grow: 1;
}
 
.scrollbox-clip[orient="horizontal"]{
  contain: none !important;
}
:root[tabsintitlebar="true"] #nav-bar{ padding-left: 20px }

#nav-bar,
#PersonalToolbar{ flex-grow: 1000; }

/* Fx <108 only, this margin on bookmarks toolbar makes absolutely no sense, but it won't work properly without it.  */
/*
.browser-toolbar.chromeclass-directories:not([collapsed="true"]){
  min-width: 100vw !important;
  margin-bottom: calc(2 * var(--tab-block-margin) + var(--tab-min-height));
}
*/
/* reduce urlbar minimum width. If urlbar would become narrower then main-toolbar wraps to second line */
#urlbar-container{ min-width: 250px !important; }

#TabsToolbar > .titlebar-spacer[type="pre-tabs"]{ display: none !important; }

.tabbrowser-tab{ margin-inline-start: 0 !important }

/* Make opened urlbar overlay the toolbar */
#urlbar[open]:focus-within{ min-width: var(--uc-urlbar-min-width,none) !important; }

/* Set this pref if you want unpinned tabs to become hidden unless selected */
@supports -moz-bool-pref("userchrome.hide-unpinned-tabs.enabled"){
  #alltabs-button{ display: -moz-box !important }
  .tabbrowser-tab:not([pinned]):not([selected]){ visibility: collapse }
}