Noticed that "reCAPTCHA" fields in various sites are not shown

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

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}acking-protection-firefox-desktop

Since today I am not able to use any search engine

Are you talking about the websites or the search engines in the browser?

@FredMcD, first - thanks for your reply!

All tracking protection is disabled. All extensions are disabled. All Cookies/Cache/Data is deleted. Troubleshooting mode - no change.

"Are you talking about the websites or the search engines in the browser?" -Both! Only Wiki and DuckDuck are functional, but that's not a big surprise, I guess...

IE and Edge - no issues at all. I'll try Chrome tomorrow, but I don't expect any difference... Unbelievable... Eventually, I'll go for a complete uninstall of FF.

PS Tried Chrome - no problems...

I constantly get the "Did Not Connect: Potential Security Issue" error.

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

Also, disable any proxy on the Settings page:

• Windows: "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type proxy and Firefox should filter to the "Settings" button, which you can click.

Select "No proxy" at the top and click OK at the bottom to save the change.

Any difference?

@FredMcD, As I said - MS Defender only. And I didn't change any settings recently.

@jscher2000, That did it. It was set on "Use system proxy settings." Never used a proxy though... Anyway, I'll investigate that later.

Thanks for the help and have a nice day both of you!