[gleenn]

I'm not sure if we're talking about the same exact service, but I'm definitely using a Yubikey with Firefox for Gmail. Not sure if it's enabled by default yet, iirc I had to go into Firefox about:config and twiddle a bit somewhere. What service(s) don't work?

[mrrsm]

I know there used to be a bug where Google services, as well as some others, used some different code to handle u2f which broke on Firefox. This has been fixed for a while now it seems so I am not sure if this issues still exists.

[jlgaddis]

Support was included starting with FF57, IIRC, although it was disabled by default -- I also enabled it at that time.

Now, in current versions of Firefox, I believe it is enabled by default (starting with FF59?).