As if it wasn’t bad enough that they want me to use a random internet service to add a keyboard to a usb wifi receiver, they have the balls to put this for Firefox users. I clicked out of pure curiosity, as I’m not even remotely interested in involving a corporate internet service in getting my keyboard connected to my computer. This is the message you get now on Logi Options software if you have a Unifying Receiver: 
For the curious: https://logiwebconnect.com
EDIT: some people on the thread have brought up that the error message being displayed for Firefox users is due to the WebUSB API not being implemented by Firefox due to security concerns. This still does not justify having to use a web app to plug peripherals to a PC.
I know Opera is Chromium based but according to statcounter Opera accounts for nearly the same % (actually slightly higher) overall desktop browser market share (Opera 4.48% : Firefox 5.96%)
Yet support for Opera but not Firefox 🗿
Disclaimer: I’m a Firefox user.
Support for Opera wasn’t a completely separate effort, it just happens to work because, like you said, it’s Chromium based. If it wasn’t, it wouldn’t be supported either…
Its because they did not had to do anything for supporting Opera. They made their app work with Chromium based browsers and Opera is happened to be one. Firefox on the other hand requires additional development to be supported.