Investigate alt-svc validation and cache eviction
After connecting to a site that sends an alt-svc onion header (such as a site fronted by Cloudflare), Tor Browser receives an alt-svc. When Firefox receives an alt-svc, it establishes a connection with it as verification that it is useful and usable. There seems to be a bug in Firefox (or Tor Browser) where this verification continues indefinitely, regardless of whether the original site is still open.
I'm not sure if this is because every response from Cloudflare's IP address site may return a different alt-srv, and Tor Browser connects with the IP address when the alt-srv connection fails or the connection cache is bypassed, therefore Tor Browser creates a very long lists of sites it should contact and verify. Or, maybe Tor Browser enters an infinite loop (or finite but sufficiently large in size) of testing the alt-srv's in its list, and never marks them as valid. I'm not sure why this is happening.
However, the most problematic and concerning result is that Tor Browser continually tries connecting with these sites long after any tabs for that site are closed.