How do I get this window to stop popping up when I open Firefox on my Mac?

I recently bought a MacBook Air and am still getting used to it. When you download Firefox for Mac, a window pops up in which you have to drag the Firefox icon into the Applications icon after it installs (or maybe before). This is the popup that keeps popping up.

I figured this would stop once Firefox is all installed, but here I am a week later and it won’t stop. It only pops up the first time I open Firefox after every time the MacBook sleeps or gets shut down. How do I get it to stop?