Currently we are facing something like 700 test failures with https-first mode enabled. We could annotate those tests in a somehow semi-automated fashion and then enable https-first in Nightly.

Backed out (bug 1835983, bug 1719271) for causing multiple failures.

• Push with failures

• Failure log for xpcshell failures

• Failure log for bc failures on browser_contextmenu_share_macosx.js

• Failure log for bc failures related to network

• Failure log for bc failures on browser_autocomplete_enter_race.js

• Failure log for gv-junit failures

• Failure log for mochitest-plain failures

• Failure log for fenix ui test failures

• Backout link

To see all the failures that were caused by this and to get a better overview on how this affected the tests, please visit this link where I made a filter that shows all the failures classified with fix by commit after this backout.
Thank you!

Backed out for causing mochitests failures.

• Backout link
• Push with failures
• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | toolkit/components/extensions/test/mochitest/test_ext_webrequest_worker.html | Should be expecting a request for

Also these mochitests failures.

• Failure Log

L.E. There are also these mochitests ( Failure Log ) and these build bustages ( Failure Log ).

Backed out for causing mochitests failures in browser_crossGroupOpener.js.

• Backout link
• Push with failures
• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | docshell/test/browser/browser_crossGroupOpener.js | Test timed out -

Please also check this failure.

• Failure Log

https://hg.mozilla.org/mozilla-central/rev/630819c98ef6

:mjurgens could you consider nominating this for a release note? (Process info)
We could include it in the nightly release notes.

Release Note Request (optional, but appreciated)

[Why is this notable]:
In Nightly, HTTPS-First will upgrade all top-level requests (like visiting a url in the address bar or clicking a link) to use HTTPS, if the site supports HTTPS. This could confuse users who do not know about that behavior.

[Suggested wording]:
When loading a page, Nightly will now automatically try to use a secure HTTPS connection whenever possible. This behavior is known as "HTTPS-First" or "HTTPS Upgrades", and has already been the default in Firefox's private browsing mode for quite some time. Here is a blog post from back when we introduced this feature in private browsing that goes into more detail.

[Links (documentation, blog post, etc)]:
Old blog post about enabling HTTPS-First in PBM: https://blog.mozilla.org/security/2021/08/10/firefox-91-introduces-https-by-default-in-private-browsing/
E-Mail about enabling HTTPS-First in Nightly: https://groups.google.com/a/mozilla.org/g/dev-platform/c/yt6Kc8cAHag/m/90N-MtFrAAAJ

Also:

[Affects Firefox for Android]:
Yes

(Bugzilla seems to eat away this part when its posted as normal text, very strange)

Thanks, added to the Fx129 nightly release notes, please allow 30 minutes for the site to update.