1. Summary

I use many addon buttons in Firefox.

Addon buttons

I don't find, how I can add additional bar for these buttons in Firefox Quantum.

I need add new bar, not modify already existing Menu and Bookmarks bars.

2. Not helped

  1. In previous Firefox versions I use Status-4-evar plugin, but it not Quantum-compatible.
  2. I don't find Quantum-compatible in https://addons.mozilla.org site; for example, not compatible New Add-on Bar or The Addon Bar plugins.
  3. I don't find, how I can solve this problem, in Google, for example, similar question.

3. Do not offer

Please, do not offer:

  1. use another browser,
  2. downgrade Firefox.

This should be possible with userChrome.css. I found this Github where there are a lot of customizations for Firefox available: https://github.com/Aris-t2/CustomCSSforFx

One that stood out was addonbar_extra_bookmarks_toolbar_below_navbar_fx65.css. If you add its code to your userChrome.css file, it will add another bar below the navigation bar.

/* create an empty space below navigation toolbar */
/* Firefox Quantum userChrome.css tweaks ************************************************/
/* Github: https://github.com/aris-t2/customcssforfx ************************************/

#main-window:not([customizing="true"]) #nav-bar {
  padding-bottom: 26px !important;

/* reduce maximum width of location bar and search bar on navigation toolbar */
#main-window:not([customizing="true"]) #nav-bar :-moz-any(#urlbar-container, #urlbar, #search-container, #searchbar)  {
  min-width: 50px !important;

/* move bookmarks to empty space below navigation toolbar, if on navigation toolbar */
#main-window:not([customizing="true"]) #nav-bar #personal-bookmarks {
  position: absolute !important;
  bottom: 1px !important;
  min-width: 0 !important;
  width: auto !important;
  max-width: 100vw !important;

/* reduce size of bookmarks 'more bookmarks' menu chevron and remove its background color on navigation toolbar */
#nav-bar #personal-bookmarks #PlacesChevron .toolbarbutton-icon {
  min-width: 0px !important;
  min-height: 0px !important;
  width: 16px !important;
  height: 16px !important;
  padding: 0 !important;
  margin: 0 !important;
  background: unset !important;
  box-shadow: unset !important;

/* set minimum height for bookmarks, when on navigation toolbar */
#nav-bar #personal-bookmarks toolbarbutton.bookmark-item:not(.subviewbutton) {
  min-height: 24px !important;

/* adjustments */
#nav-bar #personal-bookmarks {
  min-width: 40px !important;
  width: auto !important;
  max-width: 50vw !important;

Now you will have to move the Bookmarks Toolbar Items into the navigation bar using Firefox's customize option. Then you can use the bar just for add-ons, just drag them down. Customize

I only have one add-on on this browser so it's not a great test but I can test more once I get home.