After upgrading my distribution from 11.04 to 11.10 to 12.04 when starting a Firefox instance (from Alt+F2, not having a fixed launcher element) the icon displayed for it in the launcher and application switcher is the generic one (grey rectangle with a question mark in the middle). Why is that and how can I get the Firefox icon to display?
Searching the web I found this lead pointing me in the right direction. Then I did the following:
Lock icon to launcher (right-click on the launcher icon)
In a terminal execute gsettings get com.canonical.Unity.Launcher favorites which displayed ['nautilus-home.desktop', '/home/<USER>/Desktop/ff.desktop']
Identify relevant desktop file, in my case /home/<USER>/Desktop/ff.desktop
Have a look at the file, less /home/<USER>/Desktop/ff.desktop and check where Icon* keys point to, e.g. Icon[en_US]=/usr/lib/firefox-10.0.2/icons/mozicon128.png
My problem was that these keys pointed to locations that didn't exist any more.
Use tab-completion to determine which Firefox directory actually exists, ls /usr/lib/fir[TAB][TAB], finding /usr/lib/firefox/
Find a suitable icon in that directory find /usr/lib/firefox/ -name "*.png", finding /usr/lib/firefox/browser/icons/mozicon128.png
Change all Icon pointers to the existing icon /usr/lib/firefox/browser/icons/mozicon128.png
Close and restart Firefox, now with a Firefox icon in the launcher and application switcher
