There's an extension: https://addons.mozilla.org/en-GB/firefox/addon/u2f-support-a...
Works perfectly for me.
Works perfectly for me.
Unfortunately, it seems that they've failed with shipping U2F support in Firefox at the time, so the browser support seems to be coming to 58 (currently in Nightly). So, your options in November will be either Firefox ESR or switching to Firefox Beta.
EDIT: Source: https://github.com/prefiks/u2f4moz/issues/59#issuecomment-32...
EDIT2: My previous coworker also discovered some high-CPU issues with that add-on on Ubuntu, so be aware of that as well: https://github.com/prefiks/u2f4moz/issues/51