I know Firefox has a small market share, but this is the sort of feature other browsers may adopt. Maybe not the big boys like Chrome or Edge, but I could see all the niche privacy focused browsers implementing it and maybe even Safari given Apples claims to support user privacy. If a certain percentage of browsers started to use similar functionality I could tracking companies starting to develop countermeasures.

In fact I've already encountered one site that gave me a popup telling me to enable third party cookies. It was one of those dodgy sites that scrapes and copies Stack Overflow content and the JavaScript that enabled it was very clunky - but it worked. I'm surprised there aren't more websites already doing something similar.