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