Hi, I have installed all QR code on the Firefox Add-Ons Store, and find this is the only one suitable for me, thanks for the developing for six years.
But I have some suggestion:
1. The icon in the address bar is mainly green, makes it too eye appeal (since other things in address bar is black, white and grey), I am wondering if you can redesign the icon in address bar (maybe directly use an open-source QRCode icon).

2. The text (under the generated QRCode) has white background, it can be good when in a light theme, but when it comes to a dark theme, it does not look harmonize. Maybe the text under the QR Code can show differently depends on Firefox's theme. For example: white text color with black background when dark theme, and black text with white bg when light theme detected.

Simple, easy to use, convenient add-on. Thank You!