The next library is out, and this time allows you to control an ultrasound sensor like this one:

In this package you will find the modified Firmata you need to push to your board (UltrasoundFirmata.ino) and the blocks library for Snap4Arduino (Ultrasound-library.xml). Just File→Import it and you’re done: Ultrasound-Snap4Arduino.zip.

In the zip archive you’ll also find an example for Snap4Arduino that graphically shows the distance between the sensor and an object:

Happy measuring!