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",
      },

After reading the code above, looks like entering "75*2" also works, as it also results with "150". So, any calculation that returns "150" will trigger this easter egg.

See also:

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

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

Works for me.