Dns issues. Had similar with pihole before I configured it correctly.
Firefox
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Before you do anything else do a full memtest on your RAM. Bad RAM sectors can affect apps in weird, seemingly unrelated ways.
No idea if it's related, but I see similar behaviour (the not loading, rather than the error message) whenever Firefox requires a restart for an update. It doesn't make it clear this is what is happening, it just stops loading web pages in existing tabs. Only if I open a new tab does it show the "Restart to keep using" message.
I've spent far too much time diagnosing network issues without realising I just needed a restart :)
Yeah, I have also experienced this. Particularly annoying when I’m at work, waiting on emails to come in. Then I realize my email has been frozen in the background for 45 minutes, because Firefox silently froze all my tabs and needs to restart for an update.
I'm going to go out on a limb and say it is DNS
But it is never DNS. ^It^ ^was^ ^dns^
This failure essentially means there's an issue at either the DNS or TLS layers. I'd start looking at TLS, namely trusted root certs and OpenSSL.
I don't have time to browse all the tried solutions but this happens to me when my DNS gets wonky, especially systemd-resolved with dnssec enabled sometimes just stops resolving random domains, even with allow-downgrade.
Same. Otherwise it's dnscrypt on the router that's gone wonky.
Have you tried a hard refresh (Ctrl+F5) when this happens?