According to http://gpo.zugaina.org/ firebird is provided by bgo-overlay.  I 
don't know if this overlay is checked by gentoo devs for the quality of its 
contents or not, but you could take a look.  Using an overlay will allow your 
system to know what files were installed by emerge.  I expect your 'revdep-
rebuild' behaviour will correct itself thereafter.

If the above overlay is not good, then you can create your own local overlay 
for this purpose.

This overly is generated from ebuilds uploaded by users to bugs to bugzilla (bugs.gentoo.org - hence the name bgo), see here http://bgo.zugaina.org/

Viorel