A report by security firm Secunia found that Firefox was the most vulnerable browser last year, a total of 115 vulnerabilities were found in Mozilla's popular browser. By comparison, IE had a 31, Opera 30 and Safari 32.
One of the main reasons why so many flaws are found in Firefox is because this browser is open source:
In Mozilla's defence, there could well be a reason why its browser appears to have had far more vulnerabilities than competing packages: Firefox, unlike the others analysed by Secunia, is open source. This allows researches unprecedented access to the internal workings, and makes it far easier to spot and exploit vulnerabilities in the code. It also makes such bugs and their respective fixes uniquely public – where it is easy for Microsoft to quietly fix several bugs in a single patch and tot up only one vulnerability report, Firefox patches outline each problem that is solved and generate multiple vulnerability reports each time.