Drag to other day in weekly calendar is not available on firefox

[iKoru]

Version

1.3.0, even in official example pages(https://nhnent.github.io/tui.calendar/latest/tutorial-example01-basic.html)

Development Environment

MacOS(High Sierra), Windows(7, 10), whatever.. on Firefox(61.0.1, 59.0.2, ...)

Current Behavior

In weekly calendar, drag to other day behavior is not available. It's just stucked in the origin day and moving up and down along with the vertical timeline only. maybe bug?
(I could move the schedule through days in monthly calendar even on Firefox)
In the same environment except the browser, for instance on Chrome, I can move the schedule freely through any horizontal days.

// Write example code

Expected Behavior

It should be able to be moved through horizontal days in weekly calendar on Firefox.

[dongsik-yoo]

@iKoru Here is new version v1.6.0. Thanks.