After extracting and exploring omni.ja (just a zip file) in Firefox's installation file, it looks like to be codenamed as "tail150".

    // omni.ja/moz-src/browser/components/urlbar/UrlbarProviderCalculator.sys.mjs
    tail150: {
      style: {
        display: value === "150" && this.#sapName === "urlbar" ? "" : "none",
      },

See also:

- https://searchfox.org/firefox-main/source/browser/components...

- https://searchfox.org/firefox-main/search?q=tail150&path=&ca...