firefox css configs, goes really well with any completely black theme.

userChrome.css

#firefox-view-button {
  display: none !important;
  pointer-events: none;
}

#star-button-box {
  display: none !important;
  pointer-events: none;
}

#reload-button {
  display: none !important;
  pointer-events: none;
}

#forward-button, #back-button {
  display: none !important;
  pointer-events: none;
}

#PanelUI-button {
  display: none !important;
  pointer-events: none;
}

#navigator-toolbox {
  display: flex !important;
  flex-direction: row !important;
}

#TabsToolbar > .titlebar-spacer {
  display: none;
}

#tabbrowser-tabs {
  border: none !important;
}

#tabbrowser-arrowscrollbox {
  filter: grayscale(97.4%);
}

userChromeWindows.css

#firefox-view-button {
  display: none !important;
  pointer-events: none;
}

#star-button-box {
  display: none !important;
  pointer-events: none;
}

#reload-button {
  display: none !important;
  pointer-events: none;
}

#forward-button {
  display: none !important;
  pointer-events: none;
}

#PanelUI-button {
  display: none !important;
  pointer-events: none;
}

#toolbar-menubar {
  display: none !important;
  pointer-events: none;
}

#navigator-toolbox {
  display: flex !important;
  flex-direction: row !important;
}

#TabsToolbar > .titlebar-spacer {
  display: none;
}

#tabbrowser-tabs {
  border: none !important;
}

#tabbrowser-arrowscrollbox {
  filter: grayscale(97.4%);
}

.titlebar-buttonbox-container {
  display: none !important;
  pointer-events: none;
}

#nav-bar {
  background: transparent !important;
}

userContent.css

@-moz-document url(about:home), url("about:newtab") {  
    .search-inner-wrapper {
        display: none !important;
    }
    
    .logo {
        filter: grayscale(100%);
    }
}