I have configured DoH to use my personal NextDNS account. The account offers a log of all requests. Some domain names are requested far more frequently than I'd expect.

One example is the domain play.google.com. I generally have two Gmail tabs open (one in each profile configured to use the NextDNS account), and Gmail regularly sends requests to that domain. The DNS record has a TTL of 300 seconds, but on average I see one request every 15 seconds for that name.

Another domain name I see more frequently than expected is sync-721-us-west-2.sync.services.mozilla.com, which is the name of the Sync node responsible for my account. I often see bursts of about two dozen requests for this domain name in a very short time (about a second, but I'm not quite sure about that).

When looking at about:networking#dns, play.google.com seems to be missing from the list most of the time, in spite of constantly being resolved. The last time I checked it was listed, though. I've never seen the sync host in that list.

I don't know whether this is working as expected, but it definitely isn't working as I would expect. :) I'd expect Firefox to respect the TTL when making DoH requests, and I wouldn't expect Firefox to send many identical DoH requests in a very short amount of time.

Valentin, can you have a look?

Some of the DNS requests performed are instructed to skip the cache (which includes the DNS cache).
So when you force refresh a page, that will send a request regardless if the DNS record's TTL has not expired.
However, some of the issues presented here might be important to investigate, even if only as an optimization. I'll try to take a look soon.