TL;DR: We suggest using the Google Chrome app for device pairing
You may be having trouble pairing your device via the Android Firefox Browser due to a Firefox bug with intent filters. This prevents external apps, being the one you wish to test, from automatically opening and successfully pairing once the pair device link is opened. Firefox is working on a fix for this. In the meantime, we suggest using Google Chrome when pairing your devices for Visual Editor experiments and QA testing.
More information can be found in the links below:
For the video guide on how to pair your device - check out this tutorial.