Allow users to customize the search engine

[Rangtian]

Let the users add / config whatever search engine they want to use, just like the Edge does.

I don't understand why Firefox prevent that from happening except installing add-on.

UPDATE:

In order to show that "Add" button, you need to go to "about:config" -> "browser.urlbar.update2.engineAliasRefresh", set it to true then close the tab.