Firefox 3.0.4 blinks blank within window when compiz enabled

[Vitorio Okio]

I'm running Ubuntu 8.04 LTS. The video card is ATI Radeon 9550 Pro with 
Open Source ATI driver in use. 

The driver is configured to the best point I could make it.  There are no 
problems playing any video, User Switcher works like a charm, etc.

Everything looked fine until I've enabled Normal Visual Effects in 
Appearance Preferences applet.

Since then I am having a problem with Firefox.

1. When I maximize the Firefox window quite a wide black rectangle 
appears within the window around its content. It takes some fraction of a 
second and then the window gets normal. 

This never happens on minimizing the Firefox window.

2. When getting from Full Screen Mode to normal the window gets 
completely black for the same short period and then gets normal.

This never happens on getting from Normal Mode to Full Screen Mode either.

Nothing else is affected but Firefox.

Seems to be not a big deal but it's quite annoying.

Any ideas folks how to help the situation?