As long as the "body" element is visible, Mozilla uses 100% CPU time. When the "body" area is invisible (completely covered by another window, or Mozilla window minimized), CPU usage drops to 0.

Mozilla/5.0 (Windows; U; Win95; en-US; rv:0.9.8) Gecko/20020204
Mozilla/5.0 (Windows; U; Win95; en-US; rv:0.9.9) Gecko/20020311
Mozilla/5.0 (Windows; U; Win95; en-US; rv:1.0rc1) Gecko/20020417
Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.4) Gecko/20030624
Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.6) Gecko/20040113

Bugzilla #110113

Bugzilla #130055