David Wheeler took the time and had a closer look on the time spans in which no unpatched exploit for a couple of popular web browsers existed. His findings where somewhat devastating, especially for IE:

It turns out that there were only 7 days in 2004 that you could have somewhat safely used Internet Explorer (it was October 12-17), even assuming that attackers only used publicly-known attacks, and that you were only worried about the worst kind of attacks.