Appstream doesn't provide a firefox icon:
```
$ appstreamcli get firefox.desktop --details
Identifier: firefox.desktop [desktop]
Name: Firefox Web Browser
Summary: Browse the World Wide Web
Package: …
Below is a snippet of the accumulated workspaces on a single executor
```
...
7.3M doc-fuerte-rospkg-precise-amd64
36M doc-fuerte-ros-precise-amd64
136M doc-fuerte-ros_realtime-precise-amd64
4…