Since all browsers are affected, perhaps your security software (or parental control filter, or proxy server) is the culprit?

One thing you could try within Firefox would be to set Firefox to ignore any proxy server settings on your system. Here's how:

Open Settings using either:

• "3-bar" menu button (or Tools menu) > Settings (previously "Options")
• Type or paste about:preferences into the address bar and press Enter/Return to load it

In the search box at the top of the page, type proxy and Firefox should filter to the "Settings" button, which you can click.

The default of "Use system proxy settings" piggybacks on your Windows/IE "LAN" setting. "Auto-detect" can lead to a flaky connection. Set this to "No proxy" and then OK the change at the bottom of the dialog.

I suggest testing YouTube in a new tab. Any difference?

There was a transient YouTube issue that led to a lot of reports on other forums. Is it working for you again?

Thank you for the quick reply. The issue is indeed solved now and I'm not sure what exactly the solution was since I attempted many things. The last thing I did was a clean reinstall of Firefox which resolved the issue for Opera too.