Absolutely hideous since Firefox 57.0 launched.
It's probably something rather fundamental that has changed in Firefox that means I'm not stuck with the default tab style AS WELL as the tree style tab.

I think Piro has either reached obsolescence or lost sight of the original idea.
I wanted less screen wastage, now I've got a button, and the tree tabs have a header, *and* I've got the tabs at the top.

If I can't get rid of the regular tab style, I will just have to get used to it.

Real shame - I've been using Tree Style tabs for years, and you've been perfect - but now it's much too ugly to use.

I'm sorry. :(