When you say you lost the dock icon, do you mean there is no icon even when Firefox is running? or just that you have to browse to it to run it? If it has an icon in the dock when it is running just right click on that ('control-click' if you don't have your mouse configured to allow right clicking), go to "Options" in the contextual menu that appears and you will see "Keep in Dock". Just make sure that option is checked and it will be in there. Sorry if I appear to be teaching you to suck eggs, if the problem is more complex than that.