Hey, just because someone has not yet used the Opera web browser does not mean that he/she is an alien!
paul_007, the fundamental difference between Opera and Firefox is that while Opera is a very high featured browser and has a horde of functions right out of the box, Firefox's default installation does away with all the unnecessary (and in some cases, necessary) baggage and is basically a stripped down alternative to Opera. However, Firefox has a huge community supporting it and supports infinite customisation options by way of extensions, plug-ins and skins. Opera does offer some customisation options by way of widgets, skins and manually editing the configuration file, but that's about it. However, I prefer Opera because in a size smaller than that of Firefox, it offers a lot of features in the standard installation for which you would have to hunt a lot in the huge database of Firefox's extensions and some of them might even be missing completely. Plus, there are many features that you normally would not download from Firefox's database on free will, but once you use them in Opera, you will realise how great the feature is (an example would be mouse gestures).
For a detailed post on fifteen reasons why I prefer Opera over Firefox, you may read this post.