Allow U2F devices to be used in Firefox

Parent issue: #15337 (moved)

• With an addon: https://addons.mozilla.org/en-US/firefox/addon/u2f-support-add-on/
• I chose to keep this separate for a few reasons:
  • This might complicate feature-detection code (or browser detection, as it currently stands)
  • Not an official Firefox feature, so needs more testing, preferably on all client platforms