Very recently I wrote about a Firefox extension: the FlashTracer written by Alessandro Crugnola. This extension will only make the hearts of Flash developers go faster, which it did in my case: I love this plugin! And although this is a relative new Firefox extension, it already helped me debug a application that I build.
Alessandro created another Firefox extension: Flash Switcher extension for Firefox
Flash Switcher – This extension comes with all the currently published flash player plugins (from the version 2 to the most recent 9,0,16) and allows you to easily switch from one plugin to another, or also to remove the currently installed plugin (maybe for testing the express install).
I used to use KewBee Plugin Switcher. Which is now for free, but if I remember correctly will be a commercial program. The big plus of that program is that it also works for IE, but who cares…. (and windows doesn’t like that you install / uninstall something without their knowledge, so it creates lot of ERRORS).