Firefox and monitor powersave mode

[Mike C]

I have a desktop that sometimes stays in the screensaver and the monitor 
does not go into powersave after the prescribed time. 

I run F8 with the KDE desktop. Nothing fancy and compiz/desktop effects
are not used.

I found that if Firefox is not running then this is never a problem and the
monitor does go into powersave mode perfectly properly after the prescribed
period.

On the other hand if firefox is running then sometimes the problem occurs and
sometimes not....

Anyone else seen this kind of issue?  If so is there a workaround/fix?