Most of Firefox user base has always been on Windows, not Linux. What OS do you think the "techies" that promoted Firefox to replace IE in the first place were running?
Sure maybe 20 years ago. But back then Linux's userbase was also on Windows, because desktop Linux hadn't really become usable yet. I think nowadays Firefox's marketshare is a lot higher on e.g. Ubuntu (where it's the default) than it is on Windows (where Edge is the default).