mobile-config-firefox/chrome/centered_tab_content.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/centered_tab_content.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */
.tabbrowser-tab[selected]:not(:hover):not([pinned]) .tab-label-container,
#tabbrowser-tabs:not([closebuttons="activetab"]) .tabbrowser-tab:not(:hover):not([pinned]) .tab-label-container{ margin-inline-end: 7px }
.tab-content:not([pinned])::before{
  display: -moz-box;
  content: "";
  -moz-box-flex: 1;
}