If that's the case there would be a sudden decrease in the market share graph around 2019 (although that graph is from Statscounter which has Firefox currently

If that's the case there would be a sudden decrease in the market share graph around 2019 (although that graph is from Statscounter which has Firefox currently sitting at around 3.2%). The decline has already been happening since around 2010 without any drastic ups or downs.