We deprecated the U2F DOM API in 109 (Bug 1804579). Use telemetry (e.g. USE_COUNTER2_DEPRECATED_U2FRegister_DOCUMENT) shows little activity, and Chrome has fully deleted their support of the API. So we can set the default value of security.webauth.u2f to false in 111.

Release Note Request (optional, but appreciated)
[Why is this notable]: It is possible that this change breaks some second factor authentication flows.
[Affects Firefox for Android]: no
[Suggested wording]: The deprecated U2F Javascript API is now disabled by default. The U2F protocol remains usable through the WebAuthn API. The U2F API can be re-enabled using the security.webauth.u2f preference.
[Links (documentation, blog post, etc)]:

Backed out changeset d24c37cd2f63 (Bug 1809333) for causing mochitest failures on test_interfaces_secureContext.html.
Backout link
Push with failures <--> 2
Failure Log

I updated the failing test so that it expects the interface to be disabled.

https://hg.mozilla.org/mozilla-central/rev/8e01a52267b3

Added to Nightly release notes.
Keeping the relnote? flag open to keep it on the radar for inclusion in our final release notes.

Backed out for causing Bug 1814983.

Backout link: https://hg.mozilla.org/integration/autoland/rev/0d34245412816a5db90f8d6d25337673cd6fd33a

Removed the release note from 111, since this was backed out.

We're waiting for a change to Google Accounts. We expect to be able to land this again in 2 weeks.

https://hg.mozilla.org/mozilla-central/rev/c2c5479b5bf0

Note added back to Nightly 112