I am having similar problems with my Yubikey NEO. The card is detected fine and works correctly on Firefox ESR and Chromium on Debian stretch. Unfortunately, in Firefox from the snap repository, there is some communication failure. While normally the Yubikey blinks slowly waiting for the U2F tap, there it doesn’t seem to detect it needs to do anything. There’s no error message in Firefox or in the terminal.
I have tried reverting to earlier releases of Firefox using snap revert, back to 59, with no improvement. I have also tried to revert core, and that did not work either. Note that I had to uninstall firefox each time i did a revert otherwise the fonts would become all weird unreadable blocks.
It’s really too bad, because the Firefox snap works on my workstation, in a similar environment, which is an up to date Debian stretch.
Any advice on how to debug this in any way? it’s pretty frustrating… 