Flash displays are a mix of bimap, vector, font fragments plus code to tie them together and animate them, so there is no embedded stream of bitmaps to grab snapshots from.
To make a thumbnail you must actually run the flash object in flash player or a browser plugin, and use a screen capture or movie recorder program.
Also Flash can have a user interface (e.g. a Start button) so even running it might just give you the initial screen.
All of which is a long-winded way of saying it's a bigger job and so a tool is much less likely to exist.