Anecdotally, I've noticed an increasing number of websites where basic functionality does not work on desktop Firefox. I now have to open Chrome on a daily basis whereas just a couple years ago I was 100% FF for browsing (not counting using other browsers for front end testing etc).

For example, FedEx tracking refuses to work on Firefox. Shapeways (3D printing service) landing page was (is?) broken/non interactive on FF. Also a vBulletin based forum I regularly visit started return 400 Bad Request responses when visiting with FF. All these work fine with Chrome and used to work with Firefox.