Personally, I don't think IE is any less or more secure then Netscape, and I'm inclined to think that Netscape might start going downhill in quality since they are now owned by A-Oh-Hell.
The reason is simply that because IE has the largest share of the market, if there are any bugs, loopholes, etc., and MS is so big, that everybody hears about it, it makes the News and it gets a huge amount of publicity. Even tiny minor bugs end up being blown way out of proportion because of this.
I know of people who have found lots of security holes, bugs, errors, etc. in Netscape. Probably just as many in Netscape as in IE.
I really don't know much about the other browsers (Opera, NeoPlanet, Dr.Spy, Webcrawler, etc.) but I would suspect they are just as buggy.
In fact, I would say that probably the most secure browser out there is.......Lynx! Due to the fact it's all text-based, and is for Unix/linux and that practically nobody uses it makes it much less of a target for attacks.