UPDATE: To get it to work in FF6... Rated 5 out of 5 stars

Install the "Add-on Compatibility Reporter" by Mozilla (https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/). After that is installed and you've restarted FF6, go into your Add-On list (via Tools -> Add-ons -> Extensions) and then double click on "Download Flash and Video" to view details and then go to the button at the bottom titled "Compatibility" and select "This add-on still works." Restart FF6 again and you're all set!

This review is for a previous version of the add-on (0.32).  This user has 2 previous reviews of this add-on.