Check that the server is running

Here's a working url on windows
jdbc:firebirdsql://localhost:3050/c:\fede\test.fdb?lc_ctype=UNICODE_FSS

On Unix/Linux
jdbc:firebirdsql://localhost:3050//var/db/test.fdb?lc_ctype=UNICODE_FSS
^
See there's a double '/' here ---^

Check that /var/db has owner or group rwx permission for firebird user.
easyly done with
sudo chown firebird:firebird /var/db
sudo chown firebird:firebird /var/db/*.fdb

Good luck.