I can confirm as well that it doesn't work with Firefox 67.0.4 on Ubuntu 19.04 as well as on Arch Linux.
Also not working on Firefox 67 on Fedora 30............... Cannot register my security key :(
I too am seeing this on the latest Firefox, 67.0.4 (64-bit) and Win 10. As of 67 Firefox's U2F support is even enabled by default. But I did double check in about:config and yes it is enabled. Based on other responses here it seems github explicitly requires Chrome for U2F by checking the useragent. This is entirely unwarranted and I don't understand how github hasn't changed this. I suspect if I install a plugin to spoof my useragent to appear as chrome it would probably work but there is no reason I should have to do so.
As of today I'm still not able to add a key on Ubuntu 19.04 and Firefox 68 (where it enabled by default)...
For Firefoxyou can try this workarround:
Also works on other websites.
It's a shame that GitHub developers took the lazy way and just checked the browser instead of the API. I'm perplexed by the fact that something this simple still hasn't been fixed and that a workaround is needed.
This ends up being a pretty good workaround, as you only need to spoof the Chrome useragent to enroll the security key; I was able to turn off the useragent override after setting up my key and authenticate with Firefox/default useragent. Looks like the Chrome check is only being done on the security page. Still for shame, Github. Get it together.