Michael W. Muchmore (ExtremeTech) asks the question and gives a decent review of the 3 most popular browser in an article with the same title.
His conclusion :" If it's add-ons or extensions you want, Firefox still has the upper hand, with the most free plug-ins of any browser, but IE7 already has a decent catalog—though some cost money—and there are plenty of nifty "widgets" available for Opera 9 already. But neither of these offers the flexibility of Firefox's extensions and programmer hooks. And unlike Firefox, and Opera, Macintosh and Linux users need not apply with IE7.
These are all fine apps; we recommend you give them each a spin—it's a free download, after all—and stick with the one you find the most comfortable"