I have the same issue with FireFox. However, its works fine with Edge.
OnlyKey Firmware: v2.1.0-prodc
OS: Windows 10
Browser: FireFox 87.0 (64 bits)
However, GitHub does not show any error message. The image below shows the error with FireFox 87.0 on Windows 10. Please note that it works fine with Edge.
Note: I tried using Ubuntu 20.10 with FireFox (version 87.0 - 64 bits) and Chrome (version 89.0.4389.114 - 64 bits). The OnlyKey does not even blink blue (thus, the operation fails).
I just used the OnlyKey under Windows 10 / FireFox 87.0… and it works.
However, the use case is different from what I mentioned earlier:
In my previous post, I mentioned that I could not set up the OnlyKey as a security key for GitHub.
Thus, I’ve set up the security key using Edge.
Rigth now I was logged on my GitHub account (using Firefox), and I deleted a repository. GitHub asked me for confirmation, and then asked me to touch my security key. I touched it, and it worked out OK.
Therefore the problem seems to happen only when you try to set “declare” the OnlyKey as a security key. But, once declared, it’s OK.