They're both about the same in terms of resources. Both browsers will start to eat 90+MB of RAM if you have a bunch of tabs open. As for speed, I find that Opera is faster becaue the cache options are more extensive. In Firefox, it's basically "cache: yes or no?". In Opera, you can choose to never check images (makes very fast browsing), never check other stuff (flash?), and always check documents. With those Opera settings, browsing is very fast. When images on the hard drive are never compared to the images the server has, less bandwidth is required to load each page, and that results in faster browsing.
IIRC, you can set Firefox to never check against the server, but that results in really crappy browsing. While Opera lets you select documents, images, and other individually, Firefox groups them together. You should absolutely never set any browser to never check documents. TIMO forum pages are dynamically created, but the the page itself is an html page (look at the URL and you'll see what I mean). When you set either Firefox or Opera to never check documents, the forum never updates.
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