Add this to your UserChrome.css for the FireFox profile you use. @namespace url(...

Add this to your UserChrome.css for the FireFox profile you use.

      @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
      #TabsToolbar {
          visibility: collapse !important;
      }

      #sidebar-box {
        --bar-width: 40px;
        position: relative !important;
        overflow-x: hidden !important;
        /\* margin-right: calc(10px \* -1) !important; */
        /* left: var(--bar-width) !important; */
        min-width: var(--bar-width) !important;
        max-width: var(--bar-width) !important;
        border-right: 1px solid var(--sidebar-border-color);
        z-index: 3;
        transition: all 0.1s;
     }

     #sidebar-box:hover {
        --expanded-width: 250px;
        margin-right: calc(
           calc(var(--expanded-width) - var(--bar-width)) * -1
  ) !important;
        /\* left: var(--expanded-width) !important; */
        min-width: var(--expanded-width) !important;
        max-width: var(--expanded-width) !important;
     }

     #sidebar-box:hover #sidebar-header {
        min-width: var(--expanded-width) !important;
        max-width: var(--expanded-width) !important;
     }

    /* #sidebar-header is hidden by default, change "none"

to "inherit" to restore it. / #sidebar-header { min-width: var(--bar-width) !important; max-width: var(--bar-width) !important; overflow: hidden !important; }
/* #sidebar-splitter styles the divider between the sidebar and the rest of the browser. \*/ #sidebar-splitter { display: none; }
You might have to create that file in a Chrome folder in your profile folder if it doesnt exist. i.e.:
.mozilla/firefox/<your-profile-folder>/chrome/userChrome.css
you can find the right folder by going to about:support in the url bar and clicking on Open Directory in the page that shows in the Application Basics grid. This works for me on Ubuntu on FF 137.0.2