Earlier quoted context omitted.
Firefox has a market share around 3%. Even most technologists stopped using it long ago. Many banks and government websites don’t even support it anymore and loudly tell people to use Chrome instead, especially in developing countries. Nothing can kill Firefox, because it’s already dead for all practical purposes.
Wikimedia stats from last year put it at 15% of desktop browsers, ahead of Safari and Edge.
Global browser marketshare never made much sense. You need to figure out what your users use, then aim to be compatible for most of those, and ignore any global stats.