"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 removes saved passwords for this website as well. *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Does it still happen in a [[Profile Manager - Create, remove or switch Firefox profiles|new profile]]? An easy way to test a new profile is to install [https://aurora.mozilla.org Developer Edition] and see if it happens there or [[Refresh Firefox - reset add-ons and settings|refresh your existing profile]].