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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.
Have you tried the old "Unifying Software"? https://support.logi.com/hc/hu/articles/360025297913 Maybe try the Bolt app: https://support.logi.com/hc/hu/articles/4418089333655
Edit: the FAQ says you should be able to add devices with the Logi Options+ app: https://support.logi.com/hc/en-us/articles/1500007354982-How-to-connect-a-Bolt-device Have you tried that?
Last time I dealt with this I had to download that one to add/remove devices. It was always a separate app for some reason, maybe they just moved it to the browser?
Also about the firefox notice: I think in this case it's not fully Logitech's fault, it's not the classic lazy developers, you cannot workaround it with user agent switcher: Firefox doesn't support the WebUSB api, considering it's an usb device it should need this api: https://caniuse.com/webusb The supported browsers are same as the ones supporting WebUSB.
That's another thing that firefox deliberately doesn't support this api for security and privacy reasons: https://mozilla.github.io/standards-positions/#webusb
The next question is why they developed this app as a webapp? It doesn't make any sense. If you don't have internet you cannot setup your keyboard? There are still a lot of situations when people have to use their computers without internet, this just plain stupid. But I've seen a lot of stupid things from logitech, even though I love their hardware, typing this from my K750 solar keyboard.