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...
I switched to Firefox on Android, because I can use uBlock Origin (adblocker) extension with it and stop watching nonsense ads. It's also good for battery life and protect against some malware. Chrome on Mobile, on the other hand, doesn't allow extensions.
Yeah, that's what HN readers do. Most people don't even understand what a web browser is, or even use one at that. A staggering amount of modern web traffic is driven by a Google search widget on people's phones.
There's a completely depressing number of people who go to even familiar sites by googling them.
In World War 2 the Allies didn't win because they had the best tanks and airplanes. They won because they - the US, really - could build them much faster than Axis. It's a numbers game.
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.