Wouldn't this include all traffic? As long as it's enabled Windows and Firefox are constantly sending statistics in the background. If this is measured by # of requests, then this would make sense.
Does this mean that Firefox is sending statistics in far greater frequency than Windows? Either these stats are wrong or Firefox is sending telemetry far too frequently to rival these sites (as Firefox usage is simply not high enough otherwise).
There's no chance that telemetry on the browser with 4% market share would get you anywhere near the top 10 busiest sites.
The popular domains metric is probably measured through the 1.1.1.1 dns service and firefox recently defaulted to resolving domains through cloudflares DoH service, so popular domains may be biased toward firefox users. Browser popularity is probably less biased as it would have to be measure by HTTP requests to sites using cloudflare.
Microsoft being high up wouldn't surprise me if it included Microsoft 365 and Outlook traffic. But I just can't imagine that surpassing Google search+Drive+GMail+Maps
(I didn't have coffee and forgot that Google is indeed the #1 domain)