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

I do not use any of these... but I now tried MS Edge, and the site opens. I'll get into it again...

Sorry - it does n o t open....!

Did you check the links I posted?

Yes, but no success so far...

Hello,

• Open the Firefox Menu and select Options

• In the Options window click on the Advanced icon

• Select the Network Tab

• Click the Settings button

• In the Settings window select 'No Proxy'.

• Close the Settings and Options windows by clicking on the OK button

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

Sorry, but I can't find "Options" in my menu. I already tried "no proxy", but this doesn't work either.

Hi, i think i may have a solution for you. When i went to the website myself it was unsecure. So it may have been a security extension may have blocked you out. So it might help disabling your security extensions for now. Hope you find this helpful.

                                      lev-09

This website only seems to be working if you use the http:// protocol, so if it isn't working then you may have an extension that forces https://or otherwise force a secure connection.

• http://members.iinet.net.au/~dgreen/PlanimeterVault/index.html

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Thanks! But simply deactivating add-ons doesn't solve the problem. (I need a few days to get along with additionally starting Windows 10 in safe mode with network support...)

Hi,

That planimeter site definitely wants to run http:// not https://. So, that is probably the immediate cause of the failure to connect. But, as both browsers fail the same way, my guess is that the problem is not in any browser.

Do you have an anti-virus program or a firewall program on your computer that forces your request onto https:// ? Can you look at those programs for some kind of setting like "secure all calls" or "convert all requests" or "force ssl" or "secure redirect" or words to that effect? Maybe you can add an exception for this one website?

HTH.

Jeff

Thanks again! The only anti-virus programs or similar ones that I use are Windows Defender and McAfee LifeSave, each in the most recently updated version. Any hint where I should lok there - I didn't find anything (at first glance...)? JF

Disable McAfee. Is the problem still there?

McAfee disabled - but the problem is still there... sorry!

Okay. Now disable Windows Defender. If there is still a problem,

Start your Computer in safe mode with network support. Then start Firefox.
Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode
Starting Any Computer In Safe Mode; Free Online Encyclopedia

OK - I did the following: I started Windows in safe mode; before Defender and McAfee were opened, I tried the website - still invain. Is it important whether I change to safe mode and then disable both Defender and McAfee, or should I disable McAfee and Defender first, and only then start safe mode?

In most computers, there are several types of Safe Mode.
The primary ones used most are;

• Safe Mode: Only programs that came with the computer are started. No other programs will be started unless the user starts them.

• Safe Mode with Networking: Same as above but the network drivers (internet) will also be running.

This is a way to test if one of the other programs on the computer is causing the issue.

I called for more help.

Please take your time. I'm interested in a solution, but there's no hurry. By the way I'm in holidays June 26 thru July 6, so there will nothing happen in this time. Thanks again: JF