Firefox userChrome vivaldi hidden navbar style.

userChrome.css

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

/* hide navigation bar when it is not focused; use Ctrl+L to get focus.
   Adds a slight border and offset when it appears, similar to vivaldi.
*/
#nav-bar {
  min-height:0px !important;
}
#navigator-toolbox:not(:focus-within) #nav-bar:not([customizing="true"]):not([inFullscreen]) >* {
  max-height:0px !important;
}
#navigator-toolbox:focus-within #nav-bar {
  box-shadow: none !important;
  border: 1px solid var(--tabs-border-color) !important;
  min-height: 38px;
  height: 38px;
  overflow: visible;
  z-index: 999999999;
  margin: 5px 200px -43px;
  border-radius: 3px;
  border: 1px solid var(--tabs-border-color);  
}

/* Adds a line in the active tab color, just like Vivaldi. */
#TabsToolbar {
  border-bottom:2px solid var(--toolbar-bgcolor) !important;
}

/* Removes the line that cuts through the sidebar. */
#navigator-toolbox {
  border: none !important;
}

/* Slightly larger icons for the tab bar. */
.tab-throbber, .tab-throbber-tabslist, .tab-icon-pending, .tab-icon-image, .tab-sharing-icon-overlay {
  height:22px !important;
  width:22px !important;
}


#TabsToolbar toolbarbutton:not(:last-child) image.toolbarbutton-icon {
  border-radius:0px !important;
  height:31px !important;
  width:31px !important;
  margin-bottom:-1px !important;
}

/* Shift things to the left a bit by removing the spacer for dragging. */
.titlebar-spacer[type="pre-tabs"] {
   display:none; 
}
/* Sidebar style changes */
#sidebar-splitter {
  border-color:var(--toolbar-bgcolor) !important;
  width:0px !important;
}
#sidebar-header {
  color:var(--toolbar-color) !important;
  background:var(--toolbar-bgcolor);
}

#sidebar-header {
  margin-bottom:-1px !important;
  padding:0px 4px !important;
}