I'm not sure that will affect this. I believe that's for updates to user-installed search engines, which are distributed and updated separately from the browser. Default search engines are part of the browser code itself, so they update when the application does.

Note that Google will still be included as an option, so all you need to do is click on the search box icon to choose it.

(I'm an occasional Firefox contributor and I once worked on the search engine and add-on manager code a little bit, but I'm not an expert so take this with a grain of salt.)

It's worth noting that Google isn't disappearing from Firefox; you will still be able to manage your search engines as you have before.

But not in the "new tab" page I assume?

The "new tab" page in recent versions of Firefox uses whichever search engine you select in the search toolbar. So it's easy to switch it to Google, Yahoo, DuckDuckGo, or others.

Clicking the search drop-down and picking the engine you want takes even less time.

I was already unchecked in mine.