First, this is the Mozilla Firefox support site.
Thunderbird support is located here: https://support.mozilla.org/questions/thunderbird

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-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Hi, Right now, my best guess is that you have an add-on that is blocking access to something on the site.

If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? To start in Safe Mode, go to Help > Restart with Add-ons Disabled.

If the problem does not occur in Safe Mode, then you can disable your extensions one-by-one until you find out which one is causing the problem. For more info, visit Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

If the problem still occurs, let us know, and we'll work from there.

Fred beat me by less than a minute. ☺️

I'm not asking for Thunderbird support at this location. And I do not have any of those AV programs, only Windows 10 Defender running. I have also checked my proxy settings, and they are set to system.

Unfortunately, even in safe mode, using Firefox I am unable to post a question or respond -- I am using Chrome right now, for this post. BTW, I'm also using 64-bit Windows 10.

Hmm.. Maybe the solution is something simple, like clearing your cache and cookies.

1. Click the Library button and choose History.
2. In the History menu, choose Clear Recent History.
3. In the Time Range to clear drop-down, select Everything.
4. Click the arrow next to Details to display the list of items that can be cleared.
5. Select both Cookies and Cache.
6. Click Clear Now.

If the problem still occurs, we'll need some more info about your Firefox setup. Could you please follow the instructions in the following video to provide your troubleshooting information: https://youtu.be/8wKSowxEcbw

I have already tried clearing cache and cookies for the site, to no avail. Honestly, on a personal note, I do not understand the rationale for providing instruction by video, which takes 10 times longer than text and forces you to write down the instructions your self, anyway. Nevertheless: https://pastebin.com/cCf7Vg0D

Thank you for helping!

That information looks good to me. The only possible culprit I see there is add-ons, but you've said that the problem still occurs with add-ons disabled.

There must be some detail we're missing. Would you mind sending us a video of what you're seeing? For instructions, visit the link: https://www.howtogeek.com/183231/how-to-record-your-desktop-and-create-a-screencast-on-windows/

Upload the video to a cloud service, like YouTube, OneDrive, Google Drive, or Dropbox, and paste a link to the video in this forum thread.
NOTE: This forum is public!

If you would rather keep it private, we can also have a zoom call if you'd like.

I just don't see what that would accomplish. When I try to post a message or question in Firefox, I get a page saying "Access Denied" and "you do not have permission". There's nothing else to see. So, we're left with the question of why Mozilla Support can be accessed by Chrome but not by Mozilla's own browser, even in safe mode. I have occasionally come across sites that will not render in Firefox, that I've had to use Chrome for (Chase Bank, for instance, will not render under any circumstances), but having problems on Mozilla's own site is unexpected and perplexing.

Hi constructivista,
I'm hoping that will reveal a detail that we're missing. Perhaps the page is coming from a different site?

If that's too much, here's another thing you could try:
Your Firefox settings and info are kept in a folder separate from the Firefox program, called your profile folder. Try creating a test profile, to see if the problem still occurs. See Profile Manager - Create, remove or switch Firefox profiles.