Was it working in a previous version of Firefox? Creating a new profile is a quick way of testing whether the problem is caused by your profile or software. An add-on may be responsible. You could also try clearing cookies and site data (you will then have to log in again) and the cache.

https://support.mozilla.org/en-US/kb/how-clear-firefox-cache https://support.mozilla.org/en-US/kb/delete-cookies-remove-info-websites-stored

https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems (also deals with hardware acceleration)

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

Thanks, Terry, but I had already cleared cache and cookies. And, yes, the URLs to other trees worked fine until just a few days ago. And since I had said I already tried safe mode, I thought it would be understood that it does not seem to be an add-on causing the problem. After I posted here, I did some more digging and reading, and read about the DNS over HTTPS. Disabling that allows the website to function as designed, but I'm not fond of disabling DNS over HTTPS on a wholesale basis to get one site to work.

You could add the site to a preference in about:config: network.trr.excluded-domains There is also the preference network.trr.mode which Mozilla says to alter with Settings but I can't see any way of doing that.

https://wiki.mozilla.org/Trusted_Recursive_Resolver#DNS-over-HTTPS_Prefs_in_Firefox https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also https://support.mozilla.org/kb/firefox-dns-over-https

Thanks, Terry. Excluding Ancestry.com seems to have done the trick. Yes, I had already read the article you linked.