Wikipedia
Flashblock
Flashblock is a content-filtering Firefox extension for Mozilla Firefox and SeaMonkey.
Flashblock allows users to prevent page elements, such as HTML object tag browser plug-ins and advertisements, from being displayed. Flashblock neither stops the download of Flash content nor does it prevent its execution completely.
The extension uses XBL and CSS to prevent elements of Silverlight, Macromedia Authorware, Adobe Director and Adobe Flash from being displayed. Flashblock does not prevent the elements from being downloaded.
Flashblock provides a mechanism that allows users to display the blocked elements by clicking on them, or by whitelisting.
Flashblock is based on a bookmarklet by Jesse Ruderman.