> For an online system with moderate number of users (maybe 100 tops) but
> large data volmes (> 100 million records in some tables, 35+ gigabyte
> FDB file), what would be the recommended backup scheme?

You can try nbackup utility if you are using Firebird 2.X.

Nbackup allow you to make incremental backups of your database, for
example, your backup scheme may looks like:
backup level 0 - every month
backup level 1 - every week
backup level 2 - every day
backup level 3 - every hour

Read the manual here: http://www.firebirdsql.org/manual/nbackup.html