Why isn’t it configured by default to use the KDE file picker? How do I manually enable it?
To complement.
Firefox doesn’t use KDE’s file picker by default because they use a different graphical toolkit: the former uses GTK+ while the latter uses Qt.
OpenSUSE’s Firefox uses it by default, though. As well as Kubuntu’s but that’s the snap version.
Can’t Manjaro apply a patch for this just like OpenSUSE?
maybe this is what you are looking for Plasma Integration – Get this Extension for 🦊 Firefox (en-US)
I remember a package that used to do the job, but it has been removed.
It should be quite easy to figure out how they achieve this.
And - without having looked at it - @zbe 's comment might be relevant.
Firefox is GTK - while you use QT/Plasma.
some effort to make them comply is needed
I could not tell the solution because I use the former - GTK
Xfce4 to be precise.