Currently the mozilla-firefox ebuilds install a wrapper script for /usr/bin/firefox replacing the symlink that ''make install'' gives you. It sets LD_LIBRARY_PRELOAD to contain /usr/lib/mozilla-firefox. Since xulrunner is always built externally as of 3.5, this is pointless, as there aren't even any .so files in that location any more.
Created attachment 202818 [details, diff] patch against www-client/mozilla-firefox-3.5.2-r2
Fixed in mozilla-firefox-3.5.3-r1