As I’ve said many times, “It will be amusing with the hacker scum discover that Firefox and other ‘Not Microsoft’ browsers are as fun to hack since they are as or more open than IE now.” This is a case where by trying to kill IE, they might be making it into the most secure as it has to defend the most.
The difference between MSIE and Firefox, however, is that there are extensive open source communities writing for Firefox whereas MSIE is closed and relegated to the hallowed halls of MS programming. Open source communities generally perform much more extensive exploit testing and plug their holes quicker. They may be fun to hack, but oftentimes the hackers are the same people who are patching the holes.
Think of it like the stories of corporations or the FBI/CIA hiring hackers who breached their networks in order to harden them to future attacks.