To close the browser , in the menus select File, and then Exit. Alternatively click on the red X in the top right corner of the Firefox window. To re-open it click on the shortcut you use to start Firefox.

It is always best to use File > Exit
That will close all open windows, so if there are still other windows open then these won't close if you use the close X on the title bar.

Also make sure that you do not have security software that is locking files in the Firefox program folder.

Thanks for the advice . That was quick in helping me . Many thanks

Thanks

Your advice has worked and its fixed the problem. Now I can continue with  installing the program.