My understanding is that the FF softtoken was intended to be temporary while they worked on their HID support. That might not be the case any longer though.
(Disclaimer: not affiliated with Mozilla; I just check in on bug 1065729 every so often.)
[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1065729#c262
[2]: https://wiki.mozilla.org/Security/CryptoEngineering#Web_Auth...
[3]: https://addons.mozilla.org/en-US/firefox/addon/u2f-support-a...