Referencing the FireFox thread that was going on earlier, here is another different story http://www.securityfocus.com/archive/1/378632 that concludes as:
It appears that the overall quality of code, and more importantly, the amount of QA, on various browsers touted as "secure", is not up to par? with MSIE; the type of a test I performed requires no human interaction and involves nearly no effort. Only MSIE appears to be able to consistently handle [*] malformed input well, suggesting this is the only program that underwent rudimentary security QA testing with a similar fuzz utility.
This is of course not to say MSIE is more secure; it does have a number of problems, mostly related to its security architecture and various features absent in other browsers. But the quality of core code appears to be far better than of its "secure" competitors.