Not working in Waterfox G3 (FF 78 base)
Already running other scripts, so just added this to my folder.
Doesn't seem to actually do anything - no extra menu or options show on the tabs. I do get a few occasional colored lines on the left side of each tab, but that is all.
It's doesn't have any settings or menu.
The color lines are CPU & memory. You can mouse hover them to get more detail.
If you don't see any error on the console, yes, that's all.
Ok, I think I have it. The lines on the tabs don't have any details when hovered over, but there is another small set of lines all the way on the right side of the tab bar that will give details if hovered over.

The location of the bar is also to the left of the favicon, as seen in this shot
The lines on the tabs don't have any details when hovered over
That is a bug.
From your image, the color was floating to the roof, which should be down to floor.
Firefox 91 ESR will out in 2 months, then I'll update. Hopefully fix some bugs.