Looks Like you tried everything.

Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Note: Check Both Folders

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

I have tried a clean installation in the past without any result.

Let's give it a try again.

Thanks.

I cannot even download Firefox inside Firefox: https://download-installer.cdn.mozilla.net/pub/firefox/releases/65.0.2/mac/en-GB/Firefox%2065.0.2.dmg is "Unable to connect".

Clean re-installation does not work.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

I re-started my computer in safe mode and add-on pages work perfectly in Firefox. I do not try "Secure websites" (I am not really sure what I should done).

Once again in "normal" mode, the issue remains.

What does it means?

That means something on the computer is creating the issue.

As a test, disable your protection programs.

https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

Disabling the anti-virus and the Internet monitor have no effect

I have also tried these solutions but they all of them have fail:

- Changing GPU driver from Nvidia to default macOs driver

- Updating Flash Player (tough I regularly make updates)

- Updating CUDA driver

- Reviewing macOs login items and remove unneccessary ones

- Checking fonts for duplicates and errors

- Removing all system, fonts, and application caches

- Re-building launch services

- Clearing DNS cache

- Changing Firefox Network preferences from No Proxy to Autodetect

- Changing DNS server

- Some solutions provided at https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently

- Solutions from https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

- Running EtreCheck and resolve some minor issues on my system.

- Testing a different version of Firefox (60.5.1 which is properly working under Tor Browser)

None of these made a difference.

How can I find what is causing this issue?

Thanks.

I called for more help.

Do all the problem URLs have the letters cdn somewhere in them? Some users have filtering (security or parental control) software that is allergic to CDN addresses, but I can't recall ever learning the name of the product(s) involved.

Not specifically. Most failing URLs are related with mozilla.org

For instance, some URLs return a "Unable to connect" page, such as:

• https://developer.mozilla.org
• https://mixedreality.mozilla.org
• https://games.mozilla.org/
• https://donate.mozilla.org/
• https://addons.cdn.mozilla.net/

Otherwise, some other mozilla-related pages are simply render wrong, such as:

• https://addons.mozilla.org
• https://getpocket.com

Of course, there are lots of mozilla pages that work properly and as rendered as expected.

There are also some other URLs (no-Mozilla ones) that are not well rendered or which cannot be reached, but I can see any pattern in these troubling URLs.

All the URLs failing in Firefox can be opened in other browsers such as Chrome, Vivaldi, and Safari so I doubt that could be a firewall issue.

However, as soon as I can (I am working now with my computer), I will try to completely remove the internet firewall from my system to test this possibility.

Thanks.

Finally, I have traced the source of my problems. It is Hands Off!, a internet monitor for macOs.

It blocks the connections to add-ons page even when it is supposely disabled. I must say that I have not any rule that can cause this (at least, I can find it).

I removed Hands Off and re-started the computer and Firefox ran fine. Once Hands Off! was restored, the issue re-emerged.

I have tested Hands Off! as a source of troubles in many occasions but I have ever uninstalled it before.

So, thank to all of you for your help. Sorry for inconvenience.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

Your solution today can help others in the future. That is not an inconvenience.

You might want to tell Hands Off! support about the issue.