Why is firefox looking for "libcanberra-gtk-module.so" and "pk-gtk-module.desktop" gtk3 module in RHEL6?

Issue

• When a user try to start firefox in RHEL6.10 from the command line, it showing following error:

$> firefox
    Gtk-Message: Failed to load module "canberra-gtk-module"
    Gtk-Message: Failed to load module "pk-gtk-module"
    Gtk-Message: Failed to load module "canberra-gtk-module"
    Gtk-Message: Failed to load module "pk-gtk-module"

• Both of these "canberra-gtk-module" and "pk-gtk-module" gtk2 modules are already installed on the system, but it is looking for gtk3 libraries:

# locate libpk-gtk-module.so libcanberra-gtk-module.so
/usr/lib64/gtk-2.0/modules/libcanberra-gtk-module.so
/usr/lib64/gtk-2.0/modules/libpk-gtk-module.so

# rpm -qf /usr/lib64/gtk-2.0/modules/libcanberra-gtk-module.so /usr/lib64/gtk-2.0/modules/libpk-gtk-module.so 
libcanberra-gtk2-0.22-3.el6.x86_64
PackageKit-gtk-module-0.5.8-26.el6.x86_64

# rpm -qa |grep -E 'libcanberra-gtk2|PackageKit-gtk-module'
PackageKit-gtk-module-0.5.8-26.el6.x86_64
libcanberra-gtk2-0.22-3.el6.x86_64