Firefox icon disappears from Dock

I am running Sierra 10.12.2. I installed Firefox 50.1.0 and it runs fine the problem is I launch Firefox from the Launch Manage or directly from the Application folder. I the launch the App then display my Dock select and right click on the Firefox icon and specify Keep In Dock. When I restart Sierra the Firefox icon is missing and a "?" is in its place. I have read other discussions saying to drag and drop the Firefox from the Applications folder to the Dock, that will work but when launched another Firefox appears in the Dock with a "." under it showing that is running. I believe that this is a problem since I have other Apps (ex. Photoshop, Lightroom, etc...) that do not have any problems staying in Dock.