There has been a small Mint update, and I mailed the problem to morningstar benelux. Which one helped, I don't know; but there were no problems today in accessing the site. Problem solved for the time being, I should say.

This looks like a website that you access via an open HTTP connection. Firefox 51+ will mark HTTP connections with a padlock with a strike through and shows "Connection is Not Secure" if you click the Control Center 'i' button at the left end of the location/address bar.

Note that this message is meant as a warning that you could be vulnerable and that it shouldn't prevent (block) you from accessing the website. You can press the ESC key to close a doorhanger with a warning message in the name and password field.

You can always first try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

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 -> Appearance
• 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

Thank you for the many tips to tackle this problem if it occurs again. The blocking pop-up disappeared yesterday as suddenly as it appeared at the end of last week. I can only tell you what did not work: no reaction on ESC; closing Firefox removes cookies, but the problem remained when restarting Firefox and going from the starting page (wikipedia) immediately to the website. The problem had disappeared before I read your elaborate answer, so I did not try clearing the cache or restarting in safe mode. I suspect that the website owner has changed something, but it is still http:. Firefox gives a warning, but it no longer blocks access to the site.