Hello,
Please contact me to my support email if the problem persist.
But in general this is a Firefox specific error that often disappears after restarting Firefox or whole PC.
If you can see it even after the restart, then I would suggest you to reinstall my add-on. But make sure to export your dials to a file first - from the Options page / Import backup tab (you can export also your thumbnails).

Best regards,
Juraj Mäsiar