As observed by StatCounter, a web analytics company...
I guess this is accurate for people who are in the web ads business, but otherwise obviously it's a bad proxy and seems likely it's undercounting a lot since Linux users tend to be savvy enough to block analytics and probably do less web browsing on StatCounter monitored sites.
I can tell you how much it is in 40000 requests from a "national representative sample" in the Netherlands obtained in the last 6 months: 393 user agents specify Linux without Android. That's just under 1%. Firefox is 2395, or 6%.
I guess this is accurate for people who are in the web ads business, but otherwise obviously it's a bad proxy and seems likely it's undercounting a lot since Linux users tend to be savvy enough to block analytics and probably do less web browsing on StatCounter monitored sites.
Eg they show Firefox as having too small to see (< 2%) market share here: https://gs.statcounter.com/browser-market-share - other data sources show 7-ish %.