The new Firefox 5 disabled a lot of my plug-ins. You can disable the plug-in compatibility check by doing the following:
Launch FF5, go to "about:config"
Right click to add a new property, make it of type 'Boolean', name: extensions.checkCompatibility.5.0
Set it to 'false'.
(Oh, and I should mention, any previously disabled plug-ins are now re-enabled. Good point, Beckley.)