Troubleshooting
Firefox
Firefox reads controllers through the Gamepad API, as other browsers do, with a few habits of its own. PadBench needs Firefox 120 or later.
It stops reading a pad
On Windows, Firefox stops reading a pad that isn't an XInput pad, such as a DualSense, while another app that reads controllers itself is in front, such as Chrome, Steam or a game, and tells the page nothing. The pad then looks frozen until Firefox is in front again. Behind other windows, such as a file explorer's, it keeps reading.
No pads at all
With resist fingerprinting on, as Tor Browser and LibreWolf have it, Firefox shows a page no controllers at all, so PadBench waits for one forever. Test in another browser, or turn the protection off while you test.
What it can't measure
Firefox has no WebHID, so no polling rate, chatter, motion, touchpad, battery or lights. It gives Switch pads a dead zone whose sticks rest at exactly 0, which hides any drift inside it. On a Mac, in Firefox 158 and earlier, a pad's state arrives in bursts every 50 ms, and a pad may come with its buttons numbered in Firefox's own order.