How come text looks a little worse on IE10 compared to FF 21.0 ? Text looks clearer on FF ?

Text is rendered differently in Firefox. Each browser tends to have their own interpretation. I was always fond of Safari for text but browser makers have upped their game. In general though, there isn't much you can do about it. You could try playing with the letter-spacing as Firefox (Gecko) differs to Webkit browsers and no doubt IE.

Thanks

To complicate matters browser rendering engines are different for Mac and PCs so your text will render a bit differently even if its the same browser.