

If Firefox is using GTK (GTK3 to be precise) to decorate the window, even in KDE, that might be some good news for him, altough I could be wrong bc I’ve never used Plasma for this long (30 days tops, long ago) to get to the point of wanting to change the title bar of anything.
In Cinnamon (6.4.7) I’m using an old workaround for cases like this. It’s still somewhat maintained. I can’t guarantee it will work with Plasma but it doesn’t hurt to try: https://aur.archlinux.org/packages/gtk3-nocsd-git
After installation, open (or create) ~/.XSession and put these two lines in it:
export GTK_CSD=0 export LD_PRELOAD=/usr/lib/libgtk3-nocsd.so.0
and reboot.
This is how I’ve been replacing GEdit’s CSD with a normal title bar, as well as Firefox’s:
Is that as original as Windows 10’s desktop which was actually a glorified Plasma 5? 🤭