Description:
firewall-applet displays a blank tray icon when started. When started in foreground it displays the message:
QSystemTrayIcon::setVisible: No Icon set
However right- and left-clicking on the blank space of tray icon works as expected.

Additional info:

* package version(s)
firewalld 0.4.3.3-1
dbus-glib 0.106-1
dconf 0.26.0-2
ebtables 2.0.10_4-6
hicolor-icon-theme 0.15-1
ipset 6.30-1
iptables 1.6.0-1
python-slip 0.6.4-2
bash-completion 2.4-1
gtk3 3.22.2+4+gc54f348-1
libnm-glib 1.4.2-1
libnotify 0.7.7-1
python-pyqt4 4.11.4-4

Steps to reproduce:

Run in terminal:
$ killall firewall-applet
$ firewall-applet &

You will see no tray icon and this message in terminal: "QSystemTrayIcon::setVisible: No Icon set"

Diagnostics:

Alternatively you can run:
$ strace -f -e trace=file firewall-applet
and you will see that firewall-applet tries to find icons in all possible icons locations but not in /usr/share/icons/hicolor/ where they actually are.

Possible reason: last update of gtk3 could break default icon lookup locations.

Workaround for Gnome:

Create symlinks for firewall-applet icons to appropriate sub-directories of /usr/share/icons/gnome where it can find them:
# find /usr/share/icons/hicolor/16x16/apps/ -name "firewall*.png" -exec ln -s \{\} /usr/share/icons/gnome/16x16/apps/ \;
# find /usr/share/icons/hicolor/22x22/apps/ -name "firewall*.png" -exec ln -s \{\} /usr/share/icons/gnome/22x22/apps/ \;
# find /usr/share/icons/hicolor/24x24/apps/ -name "firewall*.png" -exec ln -s \{\} /usr/share/icons/gnome/24x24/apps/ \;
# find /usr/share/icons/hicolor/32x32/apps/ -name "firewall*.png" -exec ln -s \{\} /usr/share/icons/gnome/32x32/apps/ \;
# find /usr/share/icons/hicolor/48x48/apps/ -name "firewall*.png" -exec ln -s \{\} /usr/share/icons/gnome/48x48/apps/ \;
# find /usr/share/icons/hicolor/scalable/apps/ -name "firewall*.svg" -exec ln -s \{\} /usr/share/icons/gnome/scalable/apps/ \;
# gtk-update-icon-cache --force /usr/share/icons/gnome