You can go to your desktop and find the drive that firefox loaded in and right click that to eject it. As long as you've dragged firefox to the applications folder, you've installed it.

Thank you Eve so much - but WHOA did this NOT work! First off, when I open Finder I'm not seeing any of the standard Folders, e.g Desktop, Documents, Pictures etc. What is this, through Users off the cliff into Cloud storage??? Well, because I appreciated your time I decided to simply take the icon of the Firefox download box shown on my Desktop and move it to the Trash. MacOS told me, "Do you want to Force Eject this item while some applications are using it?" Well Finder DID show me what was in my "Applications Folder." I saw Firefox in there so I decided it must be safe to drag and force this object on my desktop into the Trash. WRONG. The new "do-dad" that twirls around is not as nice as the old twirly-whirly. And my window open with the Firefox Browser froze. It twirled, and twirled and twirled. So I Shut Down. I'm typing this response from Safari. Tomorrow, I'll call Support at Apple to see if they can help me - to basically show me how to find things and set things up so I can easily access what I use often. My last MacBook Pro lasted 10 years without ever needing a fix. But there's this arrogance that's built-into these machines that will drive me round the bend. Thanks again!