Here is a semi-manual process that worked for me:
Create a new database using the HSQLDB database engine, and open both databases for editing, side by side.
Copy the tables over one by one using Ctrl+C/Ctrl+V. Be sure to copy definiton and data, and select all columns.
Caveat here: if one of your Firebird tables has a CLOB column, copying content will fail. When copying the table to the destination, first select Memo [LONGVARCHAR] as the column type in HSQLDB where Firebird has CLOB. This will create the definition, but no content, as LO atempts a setClob operation, which is not supported for HSQLDB. As a workaround, create an empty Calc sheet, copy the table there, select all cells and copy the data into the newly created table (use first row as column names, should be the default).
Recreate relationships between tables if needed.
Now copy over the queries in the same manner. If you have queries depending on each other, you may need to copy them in the correct order: first those which depend only on tables, then those which depend only on queries you have already copied.
Caveat here: if your queries use functions which are specific to one database engine, you will need to modify these queries. The LO wiki has a list of semantically equivalent functions which have different names in both DB engines. You’ll have to try out each query to ensure it still runs on the target database engine and returns the same results. (Some of the queries in my DB did not work after copying, but I did not investigate further as they are essentially abandonware.)
Next, copy over the forms as before. If you use SQL statements anywhere in your forms, and they include engine-specific functions, modify them as above.
Now copy over the reports. (Haven’t tried this, but should be very similar to forms.)
If your database includes macros, you will have to copy them over as well. (Haven’t tried this.)