netmarketshare collect data from their slim clientele base. A few years they had to modify their data using weighting to get inline with other websites. Other sites have more accurate data which will reflect browser usage more accurately. I recommend using wikimedia's data. It's one of the biggest sites that open up their usage to the public:

https://analytics.wikimedia.org/dashboards/browsers/#all-sit...

If you select all the browsers, you can see Firefox numbers are even more dire. going from 10% to 5% in the last 5 years.