Submitted by Obijon147 on Thu, 03/03/2011 - 11:22.
To solve this 'transparency' problem you can re-install Firefox 3.6.12 (you do NOT have to uninstall 4.0b), open the fast dial homepage and refresh all, this will re-enable the desired transparency effect, close. Re-open 4.0b and again enjoy full transparency.
It is important that you disable any 3rd party theme in 4.0b before you attempt this as it can screw up the theme in 3.6.12.
This worked a charm for me, hope it does for you guys too.