Today I fixed a problem with firefox trying to downgrade when I updated my System.
revdep-rebuild as like as emerge -u world ended in a cyclic up- and downgrad between firefox-2 and firefox-3.
Some packages like totem still require firefox-2 for using its gecko-engine. Setting the USE-Flag “xulrunner” fixed the problem for me, thus these packages use the xulrunner-library now.