Thank you for using Tab2QR. On Firefox 56 the popup window is just too narrow and is unchangable by add-on developer. I could make the QR code image smaller but that also makes scanning more difficult. This is no longer an issue on Firefox 57 since Mozilla overhauled the UI and made the popup window more spacious. I recommend putting the add-on button on the toolbar until Firefox 57. Apologies for the inconvenience.