Some days ago Axel Beckert suggested that the Mozilla preference "places.frecency.updateIdleTime" is known to causes CPU usage problems when not configured. With release 1.4.28 this preference is now set to 0. Several users already did test this change and reported back that there problems were gone.
While we are not yet 100% sure if this is *THE* fix I still like to ask everyone with problems to upgrade to 1.4.28 and give feedback (in the comments) if there it helped (let's say if you ran for 3 days without issues)!
Update: After some testing and user feedback: This was the correct solution!