On the Mac computer how do I know if I close the application
If you click the red radius button at the top left what you actually done is close the screen. Instead, on the finder bar click on the word Firefox to open the drop down menu and click Quit Firefox.
Another way you will know if the application is not fully closed is by looking at the dock located at the bottom of the desktop screen. if there is a dot under the browser application then it means that the application is still open even when you click the red raduis button.