Are there any browser utilities or plugins that allow you to easily switch with the version of the Flash player you are using?
For Firefox 3.x on Windows XP, Ubutntu Linux, and Mac OS X (Tiger and Leopard), Flash Switcher works well.
For a cross-browser cross-platform solution, you can use this python script:
http://www.mattshaw.org/projects/flash-plugin-switcher/
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 well integrated as well)
https://addons.mozilla.org/nl/firefox/addon/5044 (from mozilla.org, should be fine: p)