Tech note to self re: Firefox and the CDC

For the last couple of days, I've run into a weird situation when trying to load cdc.gov websites (such as their various COVID-19 dashboards) using the Firefox browser specifically. Instead of the page loading, I would get a page as follows:

Hmm. We’re having trouble finding that site.

We can’t connect to the server at covid.cdc.gov.

If that address is correct, here are three other things you can try:

Try again later.
Check your network connection.
If you are connected but behind a firewall, check that Firefox has permission to access the Web.

I had no problems connecting to cdc.gov websites with any other browser on my machine, so this was clearly a Firefox issue of some kind. I also had no problem connecting to any other sites in Firefox, so this was somehow related specifically to the combination of Firefox and the CDC.

After some searching through the Firefox support pages, it seems that this was caused by DNS over HTTPS; once I disabled that option, I could load the CDC pages normally.

I've no idea why things suddenly stopped working, although I'd guess there's some configuration error somewhere in one of the DNS servers Firefox is using as "trusted recursive resolvers". Anyway, I've turned off DNS-over-HTTPS for now, and all appears to be well.