hello, what security software are you using on your devices?

Hello. It's Windows Defender.

what happens if you try to load https://addons.cdn.mozilla.net/static/img/app-icons/med/firefox.png ?

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

philipp said

what happens if you try to load https://addons.cdn.mozilla.net/static/img/app-icons/med/firefox.png ?

Another error message.

please be more specific, what does the error message say exactly/what error code is showing up when you click on "advanced"?

philipp said

please be more specific, what does the error message say exactly/what error code is showing up when you click on "advanced"?

It's in russian. It says "Connection attempt failed" or sometimes "Error while establishing secure connection". And I see no way to get error code, only "Try again" button.

If you can't inspect the certificate via "I Understand the Risks" then try this:

Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate:

• chrome://pippki/content/exceptionDialog.xul

In the location field of this window type or paste the URL of the website.

• retrieve the certificate via the "Get certificate" button
• click the "View..." button to inspect the certificate in the Certificate Viewer

You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then please attach a screenshot that shows the Certificate Viewer.

Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) to see if that has effect.

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

Thank you all for your help.

bgr148 said

I was able to connect with add-on site via proxy server but it's not a solution.

If you can't get direct access, a proxy is your best choise.