Hello,

You can contact me on my support e-mail. Also make sure to check the description section with known bugs - for example that the add-on doesn't work on the mozilla pages (due to security). Also after installing the add-on you may need to refresh the pages to enable it.

Anyway I just tested it in Ubuntu 18.04 LTS using Firefox 60.0.1 (64-bit) using default values (middle button) and it seems to work fine.
In cases like this it's usually something easy and obvious, or maybe some other add-on is using the middle mouse click event...

Best regards,

Juraj Mäsiar