<p>Are there any utilities or browser plugins that let you easily switch the version of the Flash player that is being used?</p>

Found the following:
http://www.google.be/search?q=firefox%20switch%20flash%20version
http://www.sephiroth.it/weblog/archives/2006/10/flash_switcher_for_firefox.php
(seems nice)
http://www.communitymx.com/content/article.cfm?page=2&cid=6FBA7
(seems nicely integrated as well)
https://addons.mozilla.org/nl/firefox/addon/5044
(from mozilla.org, must be good :p)