There are several security issues affecting all major Web browsers that have remained unaddressed for years. The only known ways to fix these issues (adequately) is to break the Web This is a non-starter for any browser vendor looking to grow market share. The choice is clear for most vendors: Be less secure and adopted, rather than secure and obscure. Attacks targeting browsers can be broadly categorized into three distinct vectors: Attacks designed to escape the confines of the browser walls and execute within the desktop operating system below.
Source: https://threatpost.com/web-won-t-be-safe-let-alone-secure-unless-we-break-it-020410/73488/