Xubuntu 18.04: XUL <tab> and <tabpanels> elements don't properly use GTK theme appearance

[laniakea64]

Waterfox 56.1
Xubuntu 18.04 64-bit
Greybird GTK theme
new profile

Steps-to-reproduce:

1. install NoScript Classic from https://noscript.net/getit

2. open NoScript Options

Result:

For comparison, this is what it looks like in Xubuntu 16.04:

The situation with Adwaita GTK theme isn't much better.

16.04 (good) -

18.04 (bad) -

Firefox also has this problem. Other apps are not affected.