Firefox's UI may be slightly sluggish (if your computer sucks hard), but it's much easier to customize. It uses an XML language named XUL, which uses tags for various UI elements, this is then parsed to a C++ component named XULRunner. It makes porting to other OS so much easier, since only a few components are written in C++. Plus, the HTML parser is so much faster, and standards compliant. It is also superior at handling bad markup. Firefox supports many standards which haven't even been introduced for mainstream use, such as CSS3, too. Opera is a bloated piece of closed-source crap. Don't tell me otherwise, I've tried both, and Firefox is much more of a technical innovation as well as a usable browser, than Opera. Opera, for me at least, was slow in the UI, and rendered some W3C valid CSS2 and XHTML Strict blatantly incorrect.