Firefox / Thunderbird need libpangoxft from x11-libs/pango which is no longer present when emerging pango-1.20 from current portage.
swany@spiderpig ~/Desktop $ eix -s pango ... [I] x11-libs/pango Available versions: 1.16.5 1.18.3 1.18.4 (~)1.20.0 {X debug doc} Installed versions: 1.20.0(08:27:55 PM 03/24/2008)(X -debug -doc) Homepage: http://www.pango.org/ Description: Text rendering and layout library swany@spiderpig ~/Desktop $ slocate libpangoxft /usr/lib/libpangoxft-1.0.so.0.2000.0 /usr/lib/libpangoxft-1.0.so.0 /usr/lib/libpangoxft-1.0.la /usr/lib/libpangoxft-1.0.so Are you sure you don't just need to do a revdep-rebuild ?
Hmm I guess I got it... RDEPEND changed and it seems that I got a dependancy problem here during rebuild. Adding USE="X" and reemerging solved te problem.