I was directed here
https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Change_coming_to_MonoBook_skin_for_mobile_users
after emailing about the problem this morning. From that page I got to fabricator.
>> "This change only affects people who use the MonoBook skin on their mobile devices (and potentially other smaller screens like tablets)."
Um no. It is taking over all English wikipedia articles on my device, which is a 13" Macbook Pro:
The name of your device : MacBook Pro
The operating system (OS) of the device : MacOS High Sierra 10.13.4
The screen resolution of the device : 13.3-inch (2560 x 1600)
(I will note that 922 X 576 gets reported by https://www.whatismyscreenresolution.com/ which is quite wrong)
The browser you were using at the time, and if possible, its version : Firefox
59.0.2
I sent screenshots with my original email.
{F18655948}
{F18655957}