This morning I stumbled on something similar happening with Firefox as in Chrome, by accident.
On my Win 10 ASUS I have a setting in Start/Settings/System/Display which alters the size of text and apps etc. I had set this to 150% after finding out it sorted my issue with Chrome. This morning I discovered it also impacted Firefox. So I went through the process of changing this and testing running through various browser zoom percentages and the issue occurred at various percentages.
For example with a display magnification of 125% (STD on my ASUS) I have an issue with Firefox at the browser percentage of 100% however if I set the ASUS display setting to 150% zoom Firefox displays the sites correctly with browser zoom of 100%. There are numerous variations of this happening depending on the display zoom and browser zoom (On my HD ASUS I can zoom the display between 100-175%, with 125% being both the recommended and the optimal user experience). It also appears a restart between computer display zoom settings is required for the result to be replicated.