Just tested popup with FF57, it is working fine. Can you please check if you have disabled functionality from the options. To check, you can type about:addons in the address bar and go to list of all addons. There find "Hindi Dictionary" addon and click on "Options" button on the right side of addon listing. There "Disable double click option from all websites" should be unchecked.

I would love to help you in making it work again for you. Please let me know if you have still some issue.

Thanks