@SGaist
Hmm, no, I did not. It was fine for my first reply earlier this morning, now it is not. I haven't done anything like that, nor installed updates, nor changed my Firefox, or....
EDIT Yay! Told Firefox to clear the (just 2) cookies it has for forum.qt.io, and now I am back to old interface! I wonder what changed what cookie to make it mobile....
EDIT2 Nightmare time :( That held for a few minutes. Now the new "mobile" interface has returned. I didn't do anything special. And this time I'm not having luck with clearing out all the cookies. WTF? :(:(
EDIT3 Now clearing the cookies is working again. It's possible I had not closed all Firefox windows previously. I'm going to monitor this for a while before deciding whether it's solved or not....
EDIT4 Well, seems to be behaving itself so far today! I'll re-mark this as the solution, and keep my fingers crossed... :)