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If you've upgraded to Firefox 3.5 and you've been using -moz-opacity in your CSS, then you will see that the transparency or opacity (depending on how you look at it) is no longer taking effect. But don't worry, we have an explanation and a fix for you!
The reason behind this is that the official CSS3 property is called opacity. Firefox has been supporting this for a while and it is now the only way to get transparency/opacity with Firefox. The -moz-opacity property was vendor specific and not part of the standard and therefore it was deprecated in Firefox since the early versions.
After supporing the deprecated -moz-opacity property for years, the Firefox team has decided to remove it completly. The value for the property is still the same, ranging from 0 to 1 and a simple "find and replace" of -moz-opacity to opacity will fix this issue.
If you want more information on opacity, this article has plenty of great examples:
Css3 opacity: transparency of an element in css