epiphany and yelp will build against Firefox if specified in USE flags, but will build against Seamonkey otherwise - there is no Mozilla option. However, Seamonkey blocks Mozilla, and many packages still depend on Mozilla - so we've got a Catch-22 situation. Plus, Seamonkey still doesn't work properly: I've had pages where the links stop working after the first click, tabs which generate an error when closed, and tabs which can't be closed because they generate an error! Seamonkey isn't quite ready for the prime-time yet...
See Bug 137665.