It looks like you are probably running Firefox from the .dmg, the .dmg ejects automatically on restart and that is why the icon is missing.
Find the .dmg file...open it...then drag Firefox from the .dmg to your Applications folder on your hard drive. You can then trash the .dmg file. Then drag the Firefox icon from your Apps folder to the dock.
Edit: Just to clarify, you do mean from the dock right?...and not the dashboard?