It's not the default browser on most devices. I think the parent comment is stating that if you're technical enough to install a 3rd party browser, you're technical enough to pick your own search engine. So people chose Firefox and chose away from Yahoo

I think Firefox users have now reduced to geeks and some 'real FOSS' enthusiasts etc. I still use Firefox as my primary browser, but I haven't seen any non-tech person installing it for a long time now. They all just search for Chrome and download, install that. So in today's times if someone's installing Firefox I'd say they'd mostly know their way around.