......

Is this on Windows, Mac, or Linux?

The causes of this error code seem very diverse, from a proxy or VPN issue to mystery services running in Windows to filters in routers, but it seems to usually involve some "man in the middle" that needs to be removed from the connection.

Here are some recent threads, but it's hard to say whether any will provide useful insights:

• https://support.mozilla.org/questions/1287295 - ComLegService
• https://support.mozilla.org/questions/1287745 - Legendas
• https://support.mozilla.org/questions/1287434 - filtered by router

Win 10 1903. FF 78.0.2 64. Nothing from that helped.

Do Google sites work normally in Microsoft's Edge browser or other browsers on your system?

In chrome and edge also does not work.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

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

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  https://www.malwarebytes.com/
• AdwCleaner:
  https://www.bleepingcomputer.com/download/adwcleaner/
  https://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  https://www.superantispyware.com/
• Microsoft Safety Scanner:
  https://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  https://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  https://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  https://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  https://support.kaspersky.com/viruses/disinfection/5350

https://support.kaspersky.com/viruses/utility

Try Safe Mode with network - wifi module disable in safe mode, so I can't check. I use phone as hotspot, in it all sites works.

   Malwarebytes' Anti-Malware - can't load page. (PR_CONNECT_RESET_ERROR)

   AdwCleaner - nothing.

   SuperAntispyware - found only cookies.

   Microsoft Safety Scanner - can't load page.

   Windows Defender - nothing.

   Spybot Search & Destroy - couldn't find free version.

   Kasperky Free Security Scan - nothing.

   Anti-rootkit utility TDSSKiller - nothing.

Find out that www.mozilla.org also doesn't work.

And I don't see installed by me services in Task Manager. Only HP and Win tasks.

Make hotspot on different device (different sim card / phone model) - all work.

That's mean problem on my phone. But on phone all works.

Perhaps it could also indicate different Windows networking settings for the two different network connections? I used to know a lot about such things in the Windows XP days, but have not been keeping up with how to compare those in more recent versions.