Print to LPR with Debian Firefox ESR 52

Since the Debian Jessie security upgrade of firefox-esr¹⁾ Firefox on our servers stopped offering "Print to LPR" and showed only "Print to file" in the printer dialog.

It turned out that the upgrade apparently relies on GTK3 contrary to the old version that still used GTK2. Previously, we got "Print to LPR" working by adding gtk-print-backends = "file,lpr" to the file /etc/gtk-2.0/gtkrc. There is no /etc/gtk-3.0/gtkrc and running Firefox with strace suggested that system-wide GTK3 settings are read from /etc/gtk-3.0/settings.ini instead. To restore "Print to LPR" use

/etc/gtk-3.0/settings.ini

[Settings]
gtk-print-backends = file,lpr

Hint: Do not put double quotes around "file,lpr" as we did for GTK2!

For reference please note that the following warnings issued by Firefox right when opening the print dialog are misleading but harmless:

** (firefox-esr:17106): WARNING **: Failed to execute child process
"dbus-launch" (No such file or directory)

** (firefox-esr:17106): WARNING **: Couldn't connect to D-Bus system
bus, Could not connect: No such file or directory

** (firefox-esr:26127): WARNING **: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown:
The name org.freedesktop.Avahi was not provided by any .service files

These are in fact caused by the default printbackends "cups" and "cloudprint" and the lack of D-Bus. They go away if only "file" and "lpr" are used.