hi, what error message/error code are you receiving when you try to access https://addons.cdn.mozilla.net/static/img/app-icons/med/firefox.png directly?

Secure Connection Failed

An error occurred during a connection to addons.cdn.mozilla.net. Peer's certificate has an invalid signature. (Error code: sec_error_bad_signature)

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

I'm using Windows 10, but after I did a refresh I couldn't download any addons and a lot of pages stopped working. In the Network tool it flagged the addons.cdn.mozilla.net.

which anti-virus software are you using?

Avast. As far as I can tell, Firefox is whitelisted.

please try to 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/

Well it works. Should I leave it off?

yes, please - i'm not sure what kind of weirdness avast is performing there...

Hahaha, ok then. Thank you!