TL;DR: firebase-admin is not installed for your Python. Run python -m pip install firebase-admin with the same interpreter that runs your code.

```text
ModuleNotFoundError: No module named 'firebase_admin'
```

## Fix it

1. Install: python -m pip install firebase-admin. Expected: Successfully installed.
2. Verify: python -c "import firebase_admin; print(firebase_admin.__version__)". Expected: a version.
3. If it still fails, align pip and python: python -m pip --version must match which python. Expected: same interpreter.
4. Add firebase-admin to requirements.txt.

## When this applies
- import firebase_admin fails with ModuleNotFoundError.

## When it doesn't
- The import works but calls fail with the default-app error: init problem, different skill.
- You need the JS SDK: that is npm, not this.

## Compatibility
- firebase-admin 5.x/6.x; Python 3.8+.

## Why it happens
Fresh venvs and containers start empty. Note the dash/underscore split: pip install firebase-admin, import firebase_admin.

## Edge cases
- The old python-firebase package is unrelated and unmaintained; do not install it by mistake.