[PLUG] Mouse wheel scroll inconsistent in Firefox

[Dick Steffens]

Most of the time the mouse wheel scrolls a web page in Firefox. 
Sometimes, currently unpredictable, the effect of the mouse wheel is to 
change the zoom level. It's not clear what caused the change. Sometimes 
<ctrl>0 returns the wheel to scroll, but sometimes it doesn't. Sometimes 
the effect prevents getting the normal context menu when clicking on a 
running program's button. When it gets that bad, the only solution I've 
found is a hard power down.

A clue stick would be appreciated.
--
Regards,

Dick Steffens