Firefox with CSD: default GTK3 style buttons are missing since Firefox 140 update

Hi,
Recently with Firefox 140 update, the default Xfce GTK3 style is gone when using Firefox with Client Side Decoration (CSD).

Without CSD we can see the Matcha-dark-sea theme default buttons:

With CSD the default style buttons are missing:

Until Firefox 139 the buttons style was the user GTK theme both using CSD and non-CSD.

So my question is: is there a way to use Firefox-140+ CSD with my GTK user theme (Matcha-dark-sea)?

My guess is an incompatible theme.

I’m using the “System Theme”, not a custom theme). The description says: “use system parameters for buttons, menus and windows”.

Until version 139 Firefox rendered controls of my GTK theme correctly (dark-maia-sea, the default Manjaro Xfce theme).

I’m not the only user affected, for example:

https://www.reddit.com/r/archlinux/comments/1llw03a/firefox_140_not_using_breezegtk_theme/

The last comment in this discussion:

https://www.reddit.com/r/firefox/comments/1liwaqv/firefox_140_esr_web_browser_is_now_available_for/
(look at the last comment:

"I’m not happy. Minimize, maximize and close buttons of my Arc dark theme is not looking the same as it did with former versions and apparently you can’t change it anymore either. WTF Firefox!

GTK control ‘rendering’ is 100% GONE."
)

and also this discussion:

Maybe other Manjaro users using the default theme on Xfce are also affected.

I am referring to the system’s theme - not the browser’s theme - in this case the GTK theme applied by Xfce