From github: https://github.com/webcompat/web-bugs/issues/125909.

<!-- @browser: Firefox 116.0 -->
<!-- @ua_header: Mozilla/5.0 (X11; Linux x86_64; rv:109.0) Gecko/20100101 Firefox/116.0 -->
<!-- @reported_with: unknown -->

URL: https://app.plex.tv

Browser / Version: Firefox 116.0
Operating System: Linux
Tested Another Browser: Yes Chrome

Problem type: Something else
Description: CORS
Steps to Reproduce:
Plex website cannot communicate with local Plex media server due to CORS blocking.
<details>
<summary>View the screenshot</summary>
<img alt="Screenshot" src="https://webcompat.com/uploads/2023/8/0299a919-63aa-4207-b747-56c5805525bb.jpg">
</details>

<details>
<summary>Browser Configuration</summary>
<ul>
<li>None</li>
</ul>
</details>

From webcompat.com with ❤️

Change performed by the Move to Bugzilla add-on.

The issue was reported via the webcompat.com reporter. Since DNS over HTTPS is a browser feature, we have moved the issue. Please feel free to move the issue to the relevant project and component.

Looking at how the plex.direct thing is implemented, I found this article: https://words.filippo.io/how-plex-is-doing-https-for-all-its-users/
It seems attempting to resolve 192-168-10-2.844cb447b90941e896bda2777b4b0832.plex.direct will return 192.168.10.2
However, by default DoH ignores local IP addresses. This can be changed by flipping the network.trr.allow-rfc1918 pref to true.