Strange behaviour of tabs following headlines (firefox)

[mgred]

There seems to be a bug in the styling of tab sets. It's visible in the current version of the documentation

Notice the small primary colored line between the first tab and the content.

It's a bit strange since this seems not to be always the case

I created a repository with the above example.

My CSS skills are totally limited, so I don't understand why this occurs but I'm willing to help to solve this.

Thanks for any feedback or help