Firebird also in version 2.5 has all technology already build in to implement a fully functional replication. We have implemented this in the largest installation for a big restaurant company with about 0.6 billion records, daily about 1 million new records and 150 locations where replicated servers are working online or offline with the back office software.
If you simply want to upload the data from your local db to a remote db, you can rent a virtual server at a provider you like, install firebird there, create a secure connection (we use ssh, but any tcp over vpn can be used). copy your local database to the remote server, if required open firewall fb port (3050 or other) and when you a low number of writes on your local database, simply implement a trigger on each table, that does the same insert/update/delete with the same values using the "execute statement on external" feature.
When your local database has higher workload, it is better to put the change data (table name and pk values) from trigger into a log table and let a second connection upload the records to the target db, where the same "execute statement on external" can be used.
this is just a hint how to do that, if budget allows, we can do it for you, but stopping the database pc in the evening seems to be only typical for smaller companies