First, you need to export the data from Firebase. You can do this by navigating to the Firebase Realtime Database console and selecting the "Export JSON" option. This will allow you to download a `.json` file containing all your database entries.

Set up a local development environment where you can manipulate and process the data. Install Python on your machine if it’s not already installed, as it will be useful for processing JSON and interacting with DuckDB.

Use Python to read and parse the JSON file. You can use the `json` library to load the data into a Python dictionary. This will allow you to easily manipulate and prepare the data for insertion into DuckDB.

```python
import json

with open('firebase_data.json', 'r') as file:
data = json.load(file)
```

Convert the parsed data into a tabular format suitable for DuckDB. This might involve normalizing nested structures into flat tables, depending on your data's complexity. Use Python libraries like `pandas` to achieve this:

```python
import pandas as pd

# Example: Convert dictionary to DataFrame
df = pd.json_normalize(data)
```

Install DuckDB on your system if it’s not already installed. You can do this via pip:

```bash
pip install duckdb
```

Create a new DuckDB database file where you will store your data. You can do this using the DuckDB Python API:

```python
import duckdb

con = duckdb.connect('my_database.duckdb')
```

Use the DuckDB connection to create a new table and insert the data from the DataFrame. You can accomplish this with the `to_sql` method in `pandas`:

```python
df.to_sql('my_table', con, if_exists='replace', index=False)
```

Ensure that your DataFrame's column names and data types are compatible with DuckDB’s table schema.

Finally, verify that the data has been transferred correctly by running simple queries in DuckDB. You can perform a quick check using DuckDB’s SQL interface to ensure the data integrity:

```python
result = con.execute('SELECT FROM my_table LIMIT 10').fetchall()
print(result)
```

This will help ensure that your data is correctly formatted and accessible in the new database.

By following these steps, you can successfully transfer data from Firebase Realtime Database to DuckDB without relying on third-party connectors or integrations.