for quite some time the ABP-icon in the navigation toolbar keeps re-appearing on every restart of Seamonkey. I use the icon in the status-bar, so I usually disable the checkbox on the ABP-settings to show an icon in the navigation-toolbar. This settings is applied successfully it seems but just on the next restart of the program the icon is back again and the checkbox has it's checkmark back.
This is the setting:

There are other Seamonkey users who reported the same problem.
Customizing the navigation toolbar and dragging the icon out of it is only temporarily successful, too.