I noticed this issue the other day while debugging a site I was developing. I thought that it was being applied through some JS that I was provided. I went digging and didnt find any mention of user-select being applied. I then started noticing it on every site I went to so that brought me to believe it's coming from the browser. I can't think of any setting that I may have enabled to make this happen, so it could be something as simple as that. This is a fresh install of Firefox Dev edition, and I haven't made any changes to the defaults. Anyone have ideas where this might be coming from?