Hi Prasanna,
From what I have seen the IE rendering engine is much more forgiving to invalid and incorrect html and render it much better than FireFox. This is not to be considered as a good thing. The forgiveness of invalid HTML/CSS in IE is a "feature" and FireFox is following the standard much better.
I use IE but I like FireFox too. I wish web-developers got better at writing proper HTML and CSS so the webbrowsers could follow the standards when they render a page.
How to write and render HTML/CSS is defined by W3C:
World Wide Web Consortium
If you are a web-developer you would know the standards by heart. If something does not work as specified in the standard in a browser you would complain on that browser instead of having to write detection code what browser the user is using and adapt to it.
Personally I have not found any reason to use FireFox over IE, even due to security reasons. I have not feelt more at risk using any of those browsers.