Why use FireFox when you can use Opera?

For the past couple of days I've been getting increasingly frustrated with FireFox because it eats up all of my memory. The tabbed browsing feature is a neccessity for me (I dare not install IE7). So I switched a couple of days ago to Opera which has all the features I need and a very small footprint. After two day, I can say nothing but wonder, Why use FireFox when you can use Opera?