What has happened with the Plex Dark forum theme?

I’ve been using the Plex Dark theme in the forums for a long time. Today it seems to have changed. All the text within messages is now black on dark grey, and fairly difficult to read.

This is how this message appears. Stylus theme is disabled.

Why stop at the Plex Forum?

This appears to be a change by Discourse. I’m seeing the same thing in other Discourse-based forums.

Hmm, apparently this isn’t just Discourse:

Seems fine. Does it do that in all browsers? Have you tried an incognito window?

Hmm, I did get a Firefox update this morning. I didn’t think of that. They must have mucked something up.