So you have noticed that flash is not working on your new Firefox 4 beta install.
You have checked that the plugin is in place and you even updated it, but still no dice.
The answer is that you need to run FireFox in 32 bit mode and forego all the advantages of 64 bit.
To do this, close FireFox. Left click on the Firefox icon in the applications folder via Finder while simultaneously holding down <control>. Hit “get info” and tick the box “Open in 32-bit mode” Close that up and relaunch. It should work.
As for reasons why this is so…its a long story that you’ll need to google.