Is it the most secure? No. But is it the most widely used?
The fact that 94% of computers with an internet browser still have IE set as their default browser tells me all I need to know. jancz3rt pointed out the advantage for web-based programers...
There are advantages and disadvantages for each, that is why they still exist. I use and continue to use IE because there are few site limitations. I can enter a site and will not be greated with error messages because my browser does support, for example, ActiveX controls or Java.
Firefox does not allow ActiveX. For security reasons it is great, but for being able to view a website with interactive web content, it sucks!