In the last decade, the proliferation of smartphones led to billions of newly "connected" people using the default browser, not knowing or caring about what a "browser" is. Firefox doesn't ship its own hardware so...
¯\_(ツ)_/¯
¯\_(ツ)_/¯
There's a completely depressing number of people who go to even familiar sites by googling them.
Edit: One could argue this isn't what the news article is about but what's losing 50M in a market of billions? A device, specially a smartphone, is a delivery mechanism for services and in this day and age when internet access (apps) has become massified, having "choice" is irrelevant as far as browser marketshare goes. I do hope Firefox continues because I quite enjoy using it but if they do fold someday I won't be surprised.