Oh, and this is a screenshot of my problem.

This could be a glitch with your third party cookie setting. Services like Google that distribute content over multiple servers may not work well using the "From visited" setting instead of the "Always" setting. If you would like to test, you can modify that setting on the Privacy tab of the Options dialog:

orange Firefox button (or Tools menu) > Options > Privacy

Make any difference?

Nope. No difference.

Note that you have privacy related extensions that might be blocking cookies.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can inspect and manage the permissions for the domain in the currently selected tab via these steps:

• Click the "Site Identity Button" (globe/padlock) on the location bar
• Click "More Information" to open Page Info
• Go to the Permissions tab
• Tools > Page Info > Permissions

Starting in Safe Mode works. Do I disable my add-ons one by one to see which one is responsible?

I've found which add-on was responsible. Problem solved.

Can you tell us which extension caused it as it helps other who read this?

Sure. It was very likely SmartVideo for YouTube with possible contribution to the problem from BetterPrivacy (though if so, I'm not sure how).