Firefox had a market share of 12.63% in June 2017 according to Netmarketshare and even managed to rise above the 13% mark in 2017 before its share fell to 9.92% in May 2018.
Google Chrome, Firefox's biggest rival in the browser world, managed to increase its massive lead from 60.08% in June 2017 to 62.85% in May 2018.
Microsoft's Internet Explorer dropped a percent point to 11.82% in May 2018 and Microsoft's Edge browser gained less than 0.50% to 4.26% over the year.
[...] Netmarketshare collects usage stats and does not get "real" numbers from companies like Mozilla, Google or Microsoft. The company monitors the use of browsers on a subset of Internet sites and creates the market share reports using the data it collects.
While that is certainly good enough for trends if the number of monitored user interactions is high enough, it is not completely accurate and real-world values can be different based on a number of factors. While it is unlikely that they differ a lot, it is certainly possible that the share is different to the one reported by the company.