Does nothing for Firefox 60+

[FixitmanArizona]

Firefox 60+ has an option to remove titlebar so the theme whows without system window decorations including titlebar ( setting is
found as a checkbox in customization tab.) This checkbox does nothing with nocsd (in Xfce/xubuntu this also requires libxfce4ui-nocsd, which depends on gtk-3-nocsd) installed, and the system titlebar appear along with the one in the theme (two close buttons, minimize buttons, etc. appear, one in the theme, one for the window manager.) Setting works when NOCSD is uninstalled.
Chrome variants seem to work fine whether nocsd is installed or not.
Xubuntu 20.04.2 xubuntu session or XFCE session.
SRware Iron85.0.4350.0 (Developer Build) custom (64-bit)
Firefox 85.0.1
I had installed this to see if Firefox behaved properly with/without it installed.

[AbakumovAlexandr]

The same here.

I think the best solution would be to implement an ability to disable no csd for specific apps while having it enabled glabally.

[AbakumovAlexandr]

Actually, everything works correctly if you manually disable the KDE title bar for Firefox by following instructions here.