This has been the case all day, (I'm on Australian time) and I would have posted this in the help desk if I had been allowed to post pictures there.
Below are two composite images that show what has been occurring today.
Image 1 - is want the L/H side of the website looks like in Firefox, at 100% magnification
Image 2 - at 90% magnification
Image 3 - at 80% magnification
Image 4 - at 70% magnification
Image 5 - at 60% magnification
Image 6 - 100% using Edge
I expect you will already have noticed the differences - the L/H menu is truncated in Firefox until you get down to 60%. This isn't easy to detect - initially the thought is that the L/H menu is lacking some options. It's only by lowering the magnification that the real problem becomes apparent.
Edge is obviously effected too, but a little less so than Firefox.