I was able to figure out how to increase the shortcuts again, I’m still looking for a way to recover pinned posts:

As of update 141, more than 8 shorcuts no longer works this way:

"Increase the number of rows with browser.newtabpage.activity-stream.topSitesRows in about:config and then increase the width of the shortcuts section with the following userContent.css:

@-moz-document url(about:home), url(about:newtab) {
  @media (min-width: 1800px) {
    .ds-outer-wrapper-breakpoint-override main {
      width: 1700px !important;
    }
  }
}

"

Per u/jscher2000 on FirefoxCSS subreddit, we can use this instead:

The page now uses grid layout. There is a hardcoded number of columns based on page width. You can tweak these rules to get more columns [on the userContent.css file]:

    /** Grid column count Override Fx141 **/
    .top-sites-list {
      grid-template-columns: repeat(4, 1fr) !important;
    }
    @media (min-width: 680px) {
      .top-sites-list {
        grid-template-columns: repeat(6, 1fr) !important;
      }
    }
    @media (min-width: 920px) {
      .top-sites-list {
        grid-template-columns: repeat(8, 1fr) !important;
      }
    }
    @media (min-width: 1080px) {
      .top-sites-list {
        grid-template-columns: repeat(10, 1fr) !important;
      }
    }
    @media (min-width: 1360px) {
      .top-sites-list {
        grid-template-columns: repeat(12, 1fr) !important;
      }
    }