I have encountered the same problem in Hex Shogi 91. It occurs not only in Firefox but in Chrome and Edge too. So, I think the browser is not the issue. It's not just the Rook. I was unable to move any piece by mouse. I tested the related games Shogi, Hexagonal Chess, and Hex Shogi 81, and none of them had this problem. I will look into this tomorrow when it is not so late.
I have encountered the same problem in Hex Shogi 91. It occurs not only in Firefox but in Chrome and Edge too. So, I think the browser is not the issue. It's not just the Rook. I was unable to move any piece by mouse. I tested the related games Shogi, Hexagonal Chess, and Hex Shogi 81, and none of them had this problem. I will look into this tomorrow when it is not so late.