Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

The problem is still there.

The "save" ("enregistrer" in french) button just on the right doesn't work either, in both normal and safe mode. It seems that there is a problem with the connexion to my youtube account. I see my avatar and name on the upper right (I'm on computer), but the left menu says that I have to connect by clicking a button... that leads to this error page: "405. That’s an error. The request method GET is inappropriate for the URL /youtubei/v1/guide. That’s all we know."

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

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

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

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Thank you FredMcD, but I tried everything in your links and it's still the same. Another bug is that I can't post comments on youtube: it just displays a red "error" under the input box.

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

In this mode everything works: I can manage my playlists and add comments on youtube with Firefox.

Then something else in the computer is causing the problem. As a test, disable your protection programs.

I already tried to disable the protection of Kaspersky Security Cloud, but no effet. Now I closed the program and it's OK! There is a problem between Kaspersky and Firefox.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems.

You should contact Kaspersky support about this.

When i received this problem & read an answer i try to detect what kaspersky module does this effect. Two of them must be turned off for normal youtube working: 1) Web-antivirus 2) Protection from net attack (sorry, its translate from russian version).