How to install Firefox on my Mac laptop
The file that you just download is called a DMG file or an Apple disk image. It's a commonly used way to distribute apps for Mac.
To install Firefox using this DMG file, double click to open or mount the DMG file. Doing so would open a new window which will show you the Firefox app bundle which would look like the Firefox icon.

You can now drag the Firefox icon to the Applications folder as shown in the screenshot above. This would install Firefox app on your Mac.
Once the Firefox.app file is done copying, you can eject the mounted DMG file and move it to the Trash.
To do so, go to Finder and use the keyboard shortcut Command + Shift + C to access all disks. Now you'd find an icon with the Firefox logo on it. It's the mounted Firefox 77.0.1.dmg DMG file.

Control + click on the disk icon and select Eject &quot;Firefox&quot; to eject the DMG.

Now you can go to your Downloads folder, or the folder where you downloaded the Firefox 77.0.1.dmg file and delete the installer file.