Steps to reproduce:

1. If you have a large monitor, reduce the size of the Firefox window.
2. Navigate to "about:preferences"
3. Scroll all the way to the bottom to the "Network Settings" section
4. Click on "Settings"

Notice that the dialog ought to have a scrollbar, but it is missing. If the entire dialog is visible, reduce the size of the Firefox window further and try again.

Hmm. Also the dialog buttons ought to be visible even without the scrollbar. But they aren't.

There's a class .doScroll that appears to be added to the wrong element now (the <window> instead of the <dialog>). That means the following rule doesn't apply:

:host(.doScroll) > .dialog-content-box {
    overflow-y: auto;
}

This selector should change to querySelector the dialog, I think: https://searchfox.org/mozilla-central/rev/923eec8d2fb8078ebc7a33a9e1ce73eac01f7446/browser/components/preferences/in-content/subdialogs.js#433

https://hg.mozilla.org/mozilla-central/rev/47a34eecc666

Reproduced with Firefox 73.0a1 (20191212215040) on Windows 10x64.
The issue is verified fixed with Firefox 73.0b4 (20200112220634) on Windows 10x64, macOS 10.15 and Ubuntu 18.04 using STR from comment 0.