Thanks @nitroalex. It does not work on the test page and I’ve activated it in Firefox. The udev rule is in /etc/udev/rules.d/ I -think- the udev rule is working since the device lights up once when plugging it in and I see the entries about the nitrokey in syslog. It also shows up in lsusb.
I’ll try the new rule and get back to you shortly.
Update: Just tried with the new udev rules you posted and there was no change in behavior. Here is a screenshot from GitHub that confirms that u2f is enabled in FireFox:
However the device never blinks, doesn’t respond to taps and after a little while the page shows an error. On my other machine it works just fine and I’ve tested a number of sites.
Output from lsusb:
$ lsusb
[...]
Bus 001 Device 010: ID 20a0:4287 Clay Logic
[...]