Double Click a ListBox item to open a browser

You can add a style to ListBox.ItemContainerStyle, and add an EventSetter there:

<ListBox>
    ....
    <ListBox.ItemContainerStyle>
        <Style TargetType="{x:Type ListBoxItem}" BasedOn="{StaticResource {x:Type ListBoxItem}}">
            <EventSetter Event="MouseDoubleClick" Handler="ListBoxItem_MouseDoubleClick"/>
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

ListBoxItem_MouseDoubleClick is a method in your code behind with the correct signature for MouseDoubleClick.