Randomly lashing out at Firefox is silly, especially at this time when it's getting so much praise for performance compared to Chrome. Firefox does indeed contain some complex, micro-optimized data structures for its core data (e.g. the CSS fragment tree and the DOM). It's just that it also contains a lot more code besides.

You wouldn't use an off-the-shelf hashtable to implement the mapping from a DOM node to its attributes. You should use an off-the-shelf hashtable to track, say, the set of images a document is currently loading. Like any kind of optimization, you optimize your data structures where it matters and you write simple, maintainable code everywhere else.

Sorry, but what is that supposed to mean? Have you looked at Chromium's (or any other modern browsers') memory usage? Firefox is timid compared to it, and always has been so. Maybe it's not due to the browser engineers' low skill level, but due to the enormous complexity of modern web? It's a separate operating system on top of your operating system.