Firebird's config file (firebird.conf) does have descriptions inside that explain everything, but sometimes they are confusing and hard to understand what should you do exactly if you don't have examples. One of such settings is ExternalFileAccess. Some people are even tempted to put Full as it is much easier than trying to guess what's the correct format. Here are the basic settings ('None' to disallow external tables and 'Full' to allow them anywhere) which you probably understood yourself:

ExternalFileAccess = None

ExternalFileAccess = Full

And here are those tricky Restrict settings:

ExternalFileAccess = Restrict C:somedirectory

For multiple directories, use something like this:

ExternalFileAccess = Restrict C:somedirectory;C:someotherdirectory

For Linux users:

ExternalFileAccess = Restrict /some/directory