To monitor and control the Circuit Playground Express, a special version of StandardFirmata, called FirmataCPx, must be installed on the Playground Express.
Installation
Since the Circuit Playground Express utilizes a UF2 bootloader, installation is quick and straightforward. To install FirmataCPx onto the Express, first, click on this link to download FirmataCPx.uf2.
Next, place the Circuit Playground Express in bootloader mode by plugging the Express into a USB port on your computer and double-clicking the reset button on the Playground Express. All the neo-pixels should light green.
Open up a file explorer tool, and you should see a USB drive called CPLAYBOOT. Using the file explorer, drag the FirmataCPx.uf2 file onto the CPLAYBOOT entry. In a few seconds, FirmataCPx should be loaded on the Playground Express. The red LED should flash several times and then extinguish after a successful load. For more information about loading UF2 files onto the Express, please refer to this Adafruit page. Even though this page addresses MakeCode, the procedure for flashing a .uf2 file is the same in all cases.