YouTube announced it will drop support for old browsers on March 13, 2010. The video sharing site says any browser version older than Internet Explorer 7.0, Firefox 3.0, Google Chrome 4.0 and Safari 3.0+ will be impacted by the change.
Users of older browsers will still be able to watch videos after March 13th, but new features that will be launched in the weeks and months ahead may not be supported in these browsers.
Surfing the Web on an old browser can be a lot like running a steam engine along the tracks of a bullet train--it may still work, but it doesn't take advantage of the speed and security of the new technology. If you see the below interstitial (messaging) appearing when you try to watch a video on YouTube, it means that you are using an older version of your browser.
In mid-2009 we started notifying users that YouTube would stop support for certain older browser versions. If you'd like the best YouTube viewing experience, we recommend that you upgrade to the most recent version of your browser.