Feedback of tab moving disappeared for FireFox

[hwshim]

For FireFox, feedback images for 4-sides disappeared.
But no problems for chrome.
Thanks.

[changgyun]

This problem is not a problem stylesheet.
For Firefox, Slowly move the tabs you have seen the feedback images for 4-sides.

Perhaps it is thought to draggle bug in Firefox.

Thanks.

[happibum]

Duplicate of #622.
We may close one of them.