Comment 1 for bug 551748

I think maybe you have 2 Firefox launchers on your system. One in /usr/share/applications/firefox.desktop and a second one, which is what you added to Docky (I do not know the path of that one).

Please tell me the path to your Firefox launcher on your dock. You can find this by looking in GConf at '/apps/docky-2/Docky/Interface/DockPreferences/Dock1/Launchers' which should contain the path to your firefox.desktop. Please post that here.

Also, you can probably fix it by doing the following:

1) remove Firefox from your dock (drag it off, release)
2) launch Firefox from a terminal
3) a Firefox icon should appear on Docky, right click that and choose to Pin it

Then it probably will work fine.