An element that takes up space but does not display anything. It is usually used to place spacing within a container. If you don't specify that the spacer has a size or is flexible, the spacer does not occupy any space. If you want a small gap, consider using a
More information is available in the XUL tutorial.
<box> <button label="Left"/> <spacer flex="1"/> <button label="Right"/> </box>
Inherited from XUL element