I have experienced a similar thing today in three games already. I am unable to make a move, the piece is short of stuck to my mouse cursor. I noticed that I can get rid of that by making a right click somewhere on the page.
I have Firefox 56.0 (64-bit) on Fedora 26 (Linux).
Well I can report that I just had the same thing happen with Chrome too: Version 62.0.3202.75 (Official Build) (64-bit) on Fedora 26 gnome3
Would really like to hear if anyone else is experiencing this. Basically what happens is: I try to make a move, I press mouse1 down and move piece, release mouse1 but the move doesn't happen. Instead the piece is kind of stuck selected as if I was still holding mouse1. I have only been able to escape that so far by pressing mouse2 somewhere on the page.