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Try this: Go to settings, General, and at the very bottom you'll see network settings. Open the settings menu for that. Now you should see a window pop up inside the browser. At the bottom uncheck "Enable DNS over HTTPS". I had the same problem a few months ago, and this seems to have fixed it.
See this article for help.
https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect