This applies to Mozilla Mail so I presume the same goes for firebird.
Mail is stored under
~/.mozilla/PROFILE NAME/SUMMAT.slt/Mail/ACCOUNT NAME
from looking around mail is stored as a single file, the file Inbox being your inbox. Hence you won't be able to just copy your dbx files accross.
One way which may work if you open your dbx and they are plain text, you might get away with copying and pasting the correct parts to you Inbox file but this may screw up and is a stupid solution really.