Private Browsing usually works with extensions disabled. If you have any installed that would be my suspicion as to where the trouble lies.

"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