I think the first task is: How to read the paradox files?
Well, there is a
lazparadox package which comes along with the Lazarus installation - go to "Packages" > "Install/uninstall packages", select it in the right list, click "Install selection" and "Save and rebuild IDE". I am not using this package because it requires an external dll.
But there is another package,
TParadoxDataset, which is self-contained. However, it allows only read-only access to the paradox tables - but this is all you need for converting to another database format. This package can be installed by the Online-Package-Manager of Lazarus.
I once built a paradox file viewer based on this package (
https://github.com/wp-xyz/ParadoxViewer), and it even has a converter to SQLite3 databases. You hopefully can get an idea from it how to extend it to FireBird.