Thanks, I think this was what I was searching. Strange that it's not appearing in my search-results.

Relevant part from the site: [..]Add-on usage measured here reflects multiple facets of browser customization, including web extensions, language packs, and themes.[..]

40% is a big minority, but not really what I would call core audience, especially when language packs and themes are also counted here. And 5 of the top 10-addons in that statistic are language packs.

Though, UBlock Origin is #1 with 9.6% user-share, and it's shown to have 10.5 million users on the store-page, which means there are at best only around 100 Million users left with Firefox on desktop? Seems worse than I thought.