For any Firefox users on Macs wanting to upgrade to version 4, we've come across a solution for the PDF issue. Whilst it's not a permanent solution, from our testing it works well and should do for most users. Unfortunately, you need to run Firefox in 32 bit mode for it to work.
After the plugin is installed, close Firefox, then select the Firefox icon in Finder. Open its properties using File > Get Info (or Cmd + I) and check the 'Open in 32-bit mode' checkbox and close the window. Open up Firefox and the PDF preview should now work.
UPDATE - 11/5/2011
We've had feedback that the 32-bit mode should be set first before attempting to install the plugin, otherwise Firefox won't let you install the plugin. Switch the steps above and it should work fine.