Ugly lower tab border with linux default theme and transparent firefox theme

[CavaleriOmar]

Hi.

Stunning CSS for Firefox, but on Linux (to be exact Ubuntu 20.04 KDE - KDE Neon, in detail), a problem with the bottom edge of the tabs. Visible here: https://i.imgur.com/WubBTkM.png

This problem occurs both with the default theme for GTK applications, and with transparent or clear times, as shown here: https://i.imgur.com/geHMfNg.png.

The problem in unknown with default dark theme: https://i.imgur.com/Kw2GXgw.png