Weird problem in squid guard - Partial access to the Internet

[KOM]

Is it a case of HTTP vs HTTPS?  Are you using Transparent mode?

[firefox]

If you mean proxy server settings
Then yes

I did not find any similar settings
in the Squid Guard tab


[KOM]

Anything that silently intercepts an HTTPS session will trigger a browser Man in the Middle attack warning unless you have distributed the pfSense server certificate to all your clients.  You can avoid this by either installing the cert on every client PC, tablet, phone, etc etc, or stop using Transparent mode.

[firefox]

Not clear to me
how the other computers work correctly ?

[KOM]

If they were going to an HTTP site then it would work without any errors.  Where were these clients going when they did/didn't work right?

[firefox]

Accesses the same sites - google.com, walla.co.il, ynet.co.il,
I did not check more sites
Because it's not my computer (do not have much access to it)
It's my sister

Update
Two computers back to work correctly
Only one causing problems (my sister computer)
And there is not at all the sites

[KOM]

It could be that for the other computers, someone added the pfSense cert to the list of Trusted certificates.

[firefox]

how can i check if certificates is pfsense ?

two computer use ubuntu and firefox are ok
one computer use win xp and crome or iexplorer not ok          <–-------- problem computer
one computer use win7 and firefox is ok

[KOM]

Run IE and go to Options - Content - Certificates.  Look for a cert from either pfSense or Electric Sheep Fencing (ESF).

[firefox]

I checked all computers (5 computers)

The certificate does not appear in any of them

[KOM]

Well then, I don't know what to tell you.  You have one computer with a problem.  I doubt it's pfSense or Squid.  The one computer that has the problem is so old, it may not have the proper Root Certificate updates from Microsoft.  Chrome and IE share the same certificate store, whereas Firefox uses its own.  Try running Firefox on the XP box and see if that is any different.

[firefox]

Try running Firefox on the XP box and see if that is any different.

I'll try

Clarification
the computer is old old
the operating system is old

Update from yesterday
But it was already too late and I turned off my computer

The computer in question was one hour ahead

I set the computer time and date accurate
And now

Only two sites inaccessible
And show the same message as before