u/CleoMenemezis explained a little better on the Reddit thread about it, but a tl;dr is:
The corners are currently drawn on CPU and it is slow, and the way it’s implemented is hacky, which could cause artifacts and made impossible to be used with themes.
Looks nice. Plus, what Firefox CSS theme are you using?
Oh, it’s firefox-gnome-theme mixed with firefox-vertical-tabs. I usually use the dark mode variant, but switched to the light one for the screenshot for a better contrast.
Thanks.