Yes, you are right. It doesn't work with Firefox 52.8.1. I opened a bug report here for you:
https://github.com/Smile4ever/firefoxaddons/issues/165

Please submit bug reports via e-mail or GitHub next time. Thank you for using Translate Now, it will be fixed soon.

Edit: It has been fixed with version 4.0.2. Please try the new version.
Edit2: did you get the chance to try it out?