I'm not sure about "invalid URL" but security certificate errors have a number of causes.

Is Google the only site where you get an untrusted connection error? If you have seen this on multiple sites, it could be related to your security/filtering software.

Do you use Avast, Kapsersky, BitDefender or ESET? If so, most likely your Firefox lost trust in your security software. This can occur after a major version update of the security software, or if you use the Firefox Refresh feature (that creates an Old Firefox Data folder on your desktop).

The reason this may be relevant is those programs intercepts your browser connections for filtering and in order to read your secure connections, they present "fake" site certificates to Firefox. You either can turn off the filtering of encrypted connections or set up Firefox to trust the software's signing certificate. If you tell us which program you use, we can search up the specific steps in earlier forum threads.

Of course, this problem also can be caused by malware, so rather that make a bunch of exceptions, it's worth pausing to check out why Firefox doesn't trust the certificates.

A unique issue affecting many Windows 10 upgraders is that they inadvertently enabled parental control on their own browsing, which causes seemingly random blockages. Try disabling Microsoft Family Safety (or not being part of a family): http://windows.microsoft.com/en-us/windows-10/turn-off-microsoft-family-settings

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

• https://support.mozilla.org/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 that you update each program to get the latest version of their databases before doing a scan.

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

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

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked