The Banner weblet is essentially a marquee - a scrolling area of text - which can be added to any pane on a web page. A banner is implemented as a <marquee> HTML element.
Banners can be used to display information, or they can be an active element in the web page which will redirect you to another webpage when clicked. They are typically used for advertising or to display up to date information which changes regularly.
Firefox has a bug in the way is displays a banner that has been given a fixed size, has a "scroll" value of "scroll" or "slide" and is placed somewhere with a text alignment of "center" (such as a table cell or attachment panel). Firefox places the "box" for the banner in the correct place but calculates the location of the content assuming the banner is left aligned. The content is then only visible while the position of the content overlaps the position of the box.