To view backgrounds just open up a new tab as you do for everyday web surfing. To open a new tab you can use one of the below methods:
Use your keyboard's CTRL+T combination. This is the recommended one becuase it is the most efficient and fast method.
From the Firefox menu bar, click on File menu and choose New Tab menu option.
When the new tab opens, rather than viewing a blank white page,
one of your installed backgrounds will be shown.
At each new tab your background will change and another one will be displayed.