I don’t have any ideas unfortunately, because Brief works for me.
I installed it for the first time today, subscribed to the Slashdot RSS feed, and Brief looks like this:
There’s the [Support] Brief thread, but you already know about it, and the developers don’t seem to be reading it anymore.
I’d suggest creating a new issue on GitHub - brief-rss/brief: RSS reader extension for Firefox
The last issue is from three weeks ago.
But the last code update is from last year…