This is the link to this bug

https://bugzilla.mozilla.org/show_bug.cgi?id=627626
 

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Can confirm that this bug can be reproduced in Firefox 4.0, and Firefox 4.0 beta 9
Installing Regis'  Fast Dial Version 2.23b3 directly frm the BUG

This Version 2.23b3 is very useful for testing purposes

If one runs FD 2.23b3 in Firefox 3.6.28 there is no transparency bug
 

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