I see this ALL the time working on customers computers. You downloaded firefox, opened the disk image (the firefox 188.8.131.52.dmg) and clicked on the application to launch it. probably added it to your dock or whatever. here's what you do. 1.Quit firefox.
2. In the mounted disk image (the thing with the fancy firefox logo) take the Firefox application and drag it onto your desktop, into the applications folder, or wherever you want, just not on that disk image.
3. if you have a firefox icon on your dock, drag it off and it'll disappear in a puff of smoke
4. eject/trash the disk images
5. double click the new firefox icon you made, wherever it was that you put it.