My user agent is: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 Safari/537.36 OPR/42.0.2393.517
Facebook and some other websites show me their mobile versions. This started to happen all of a sudden several weeks ago and in all browsers i.e. FF, Chrome, Opera! I use Opera most of the time.
My zoom is at 100%, never changed it.
It's an old desktop: Windows 7 Professional 32, ATI Radeon Xpress 1100, Dell Optiplex, Intel Pentium R