Well, to start with, I have been an avid user of Firefox since from the days it was firebird(phoenix). I have really liked the new aspects introduced in teh browser, like tabbed browsing, find as you type, pop-up blocking etc. So does that mean that it is better than the browser, IE, by the software giant Microsoft. Of course, Yes. I would not state security as the primary reason for that though many do. Rather I would say Firefox adheres better to web standards than does IE. Conforming to the web standards, really makes the web application portable across platforms(Linux, Windows, Solaris etc) and also the users will not be forced to use a single platform for viewing a webpage. And about vulnerability, all softwares are vulnerable and that cannot be taken into account for proclaiming that the software is better in any way. Even firefox had some critical vulnerabilities that were fixed later. But why I would say that firefox is better is that I has a very good user experience, and being extensions based allows to add many features that are not found in the actual browser. though extensions bloat up the browser, they are quite useful and handy in most cases, for example, the spiderzilla extension downloads the entire site, Flashgot redirects the downloads to a download manager etc. Tabbed browsing is offered by other similar browser IE-based(Maxthon, Avant Browser, etc) and non-IE based(K-Meleon, Opera etc). But apart from other things Firefox has all what the normal user expects and not bloated with unused features.