chore: remove broken link to Firefox Extension (#359)

Author: lx4r

src/components/Header.js

@@ -196,12 +196,6 @@ function Header({
               >
                 Chrome Extension
               </MenuLink>
-              <MenuLink
-                href="https://addons.mozilla.org/en-US/firefox/addon/testing-playground"
-                target="_blank"
-              >
-                Firefox Extension
-              </MenuLink>
 
               <div className="border-b border-gray-200 mx-4 my-2" />