Initial Log on to YouTube always timed out

Advice how to stop the following pop up when I attempt to log on to YouTube would be appreciated. This occurs the first time I try but every re-try is successful so it is just an irritation. I am using an up-to-date Windows 10 O/S and Firefox browser.

"The connection has timed out

An error occurred during a connection to YouTube.

The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer’s network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the web."

Any firewall/anti-virus sitting between you and the browser?

Have you tried another browser to rule out Firefox and its configuration?

Does portable Firefox do the same thing?

Have you cleared out cookies and cache?