I am experiencing the same problem. In my case, this seems to be an incompatibility with the Zoom Page WE extension of Firefox ; deactivating it makes the problem go away.
The thing is, although I have a global default zoom of 150% with his extension, I also set a site-specific zoom of 100% for here.
I suspect the zoom level may imperceptibly flicker as the site is rendered, and that the moment when the 150% zoom is applied triggers some viewport calculation code that then switches to a layout intended for small devices like phones.
EDIT : testing with a fresh profile, the mere presence of the extension triggers the bug, even if the default global zoom level remains at 100% and as such no scaling is actually done.
I have never encountered such a problem, even on responsive websites.