Due to security software changes during the upgrade, many users are finding that Firefox access to secure sites either is completely or partially blocked.

If you use Avast, BitDefender, ESET or Kaspkersky, Firefox may need to be re-setup to work with its feature of scanning all your web traffic (in order to intercept and read HTTPS requests and responses, they present "fake" certificates to Firefox).

If you turned on the parental controls (Microsoft Family Safety), that similarly can cause this issue on some sites.

Considering that only two sites are involved, I would check on the parental controls first.

Sorry, was reading too many threads at once. If you are NOT getting an SSL certificate error:

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

"3-bar" menu button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or globe icon in the address bar > More Information > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

If the sites still won't load, could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Sorry, had tried these steps before posting the query (reset, safe mode, clearing cache, cookies, etc.). FYI, I am using Avast. I tried disabling the Avast add-on in firefox too. Nothing worked.

I was not familiar with Family safety stuff (upgraded from Windows 7), so I was just checking it out. Couldn't find the right setting. Perhaps you can help me find it? TIA.

Attaching the error pages for fb and startpage

On Facebook, click the gray padlock icon in the address bar, then More Information, the View Certificate. If you check the Issued by and Certificate Hierarchy, what do you see there? I have attached a screen shot for comparison.

Try to rename the cert8.db file (cert8.db.old) and delete the cert_override.txt file in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file. Otherwise you can rename (or copy) the cert8.db.old file to cert8.db to restore the previously stored intermediate certificates. Firefox will automatically store intermediate certificates when you visit websites that send such a certificate.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

@cor-el, That didn't help :(

@jscher2000, This is interesting. fb does not have a certificate problem. It does not store cookies! Any idea why? Screenshots are attached. Also, attached is the startpage's info. I noticed that the certificate is missing for startpage. Strange. TIA.

Uninstalled Avast and everything works perfectly fine! Guess we need to wait for all apps to work properly with Windows 10.

you could also disable https scanning within avast:

1. Open the Avast dashboard on the affected system.
2. Select Settings from the left sidebar menu.
3. Switch to Active Protection.
4. Click on Customize next to Web Shield.
5. Uncheck the "Enable HTTPS Scanning" option and click ok.

http://www.ghacks.net/2014/10/31/avasts-https-scanning-interferes-with-firefox-and-other-programs/

@philipp, I reinstalled avast after reading your response. Had to download the latest version to reinstall. And all sites load without making any changes! Have also installed avast add-on in firefox. All is well with the world again..! Thanks to all of you for taking the time out to help me. Appreciate it :)