Make sure you are not blocking content.

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-firefox-did-not-connect

https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors

What do the security warning codes mean

Apparently these are canned answers that are triggered by certain words. They are absolutely not responsive to the portions of my question that mentioned that I had tried Troubleshooting Mode and had temporarily turned off Windows Firewall and Avira Antivirus, as well as deleting cache, cookies, and history. So unless someone has an answer that actually addresses my question, I will continue to seek real solutions. Thanks for nothing.

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Again, not helpful. Do people read the questions or are they just scanned for key words that generate an automated response? As stated explicitly in the question, I deleted the history, so there is no "history" entry to right-click upon, and therefore "Forget" cannot be invoked. Artificial yes, intelligence ... questionable.

If you remove the history then you only clear the visited history entries. There can be a lot more website related data stored that can only be removed by "Forget About This Site". If you revisit that website again and are able to visit their pages normally then your issue is fixed, otherwise new history should be created where you can use "Forget About This Site". It is always hard to tell what specific the website doesn't like about your request (HTTP response headers) since the website doesn't give these details and it can be anything including your IP if this IP is in a range that has been flagged as suspicious in the past. Using a proxy or VPN also can be a cause.

Once again, there is no history file to click on and invoke "Forget," at least using the method described in previous "answers." And, as stated originally, my VPN was turned off. So I'm still up shit's creek.

Can you provide a screenshot of the error message and an url?