"You don't have permission to access..." errors are characteristic of a firewall that protects the server against misuse and this firewall thinks something is suspicious about the HTTP request. If you use a proxy or VPN, try to use a direct connection. Select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. *Settings -> General -> Network -> Connection -> Settings You can try to clear the Cache and remove the Cookies for websites that cause problems via the padlock button or via "3-bar" Firefox menu button (Settings) to see if that fixes it. *https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can I've seen cases where having enabled Resist Fingerprinting was causing this issue. *https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting Start Firefox in <b>Troubleshoot Mode</b> to check if an extension is causing the problem. *https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode If this all doesn't work, try "Forget About This Site" in the right-click context menu of an history item. Note that this might remove saved passwords for this website, so make a note if you have any. *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Does it work with a full URL to a public page (no sign-in required)? For example, New Releases: https://www.thegreatcourses.com/courses/sort-by/news_from_date/sort-direction/desc?new_release=1&ICMP=126573 Or is the problem when you try to access your account or library, or other pages that require a sign-in?