Hi All
Just ran into this exact same problem under the new Bionic Bever Beta 18.04. (Running KDE as well.) I struggled for a little while as Firefox was the default application that the sign in web page was appearing in.
What I discovered is that I had to go into default applications and manually set me web browser to be firefox. It was grayed out with the default option of letting the type of URL define the application. Once I switched to manually setting everything to open in firefox it was all good!!. Hopefully someone is writing instructions on how to install in the different env