Thanks for pointing it out! It came out an extension(India Rail Info) causing this.

I have observed the same in my colleague's macbook as well. It could be some of the add-on causing this.

But surely, I need to dig down further to find the cause.

I've seen this too. However, they seem to have made improvements in Firefox 15. Previously, Firefox would take around 3% CPU with just the start page open; visiting a page with an animated GIF would push it to 12%; and, most troubingly, CPU usage would stay around that level even after the page was closed. Now, the same page still takes around 11%, but Firefox stays at 0% with just the start page open, and it goes back to 0% once you close other pages.

For me its firefox extension causing it. If anybody facing this kind or problem try disabling India Rail Info App extension.