The security problems with IE only exist as most people simply will not adjust the security from its defaults. If you set it up properly, it is just as secure as Firefox is.
The biggest reason Firefox is *supposedly* more secure is due to the lack of ActiveX support. Guess what? You can disable that in IE.
People who complain about the security problems with IE simply have not taken the time to adjust the security settings available to them in it.
From where I sit, IE implements the HTML 4.0 spec better than Firefox does.