I can confirm this bug running Intrepid and Firefox 3.06 (Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.0.6) Gecko/2009020911 Ubuntu/8.10 (intrepid) Firefox/3.0.6).
Disabling "Legacy Fullscreen Support" in compiz-config settings manager worked for me.
I can confirm this bug running Intrepid and Firefox 3.06 (Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.0.6) Gecko/2009020911 Ubuntu/8.10 (intrepid) Firefox/3.0.6).
Disabling "Legacy Fullscreen Support" in compiz-config settings manager worked for me.