The rollout (Bug 1885509) is currently active for 5% of Firefox 138 release (and 100% of beta). There are no reported web compat bugs so far.

Flip the pref layout.css.h1-in-section-ua-styles.enabled to false for all channels, ride the trains for 140.

https://hg.mozilla.org/mozilla-central/rev/387293b61dc0

When this reaches stable, https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Experimental_features#ua_styles_for_h1_nested_in_sectioning_elements is no longer experimental.

Release Note Request (optional, but appreciated)
[Why is this notable]: This change was previously a desktop-only rollout. Now it affects all platforms.
[Affects Firefox for Android]: Yes
[Suggested wording]: See https://www.mozilla.org/en-US/firefox/138.0/releasenotes/#:~:text=Firefox%20now%20applies%20a%20uniform%20user%20agent%20%28UA%29%20style%20to%20%3Ch1%3E%20elements%2C%20regardless%20of%20whether%20they%20are%20used%20inside%20%3Carticle%3E%2C%20%3Caside%3E%2C%20%3Cnav%3E%2C%20or%20%3Csection%3E%2E%20%28Learn%20more%29
[Links (documentation, blog post, etc)]: https://developer.mozilla.org/en-US/blog/h1-element-styles/

Added to the Fx140 nightly release notes, please allow 30 minutes for the site to update.
Keeping the relnote-firefox flag as ? to keep it on the radar for inclusion in the final Fx140 release notes.