I used Firefox back when it was still called Firebird. It's a good browser, and the developers really do care about web standards, but the bloated, cross-platform XUL is always going to be like a hemorrhage to RAM. If the browser made use of the native API of whatever system it was compiled on, it would run a lot more efficiently. Mozilla realized this with OS X, at least; which is where Camino comes in.
Also, tests on 3.x have shown Firefox using the *least* amount of RAM of all the major browsers (including Opera), so the talk of Firefox being bloated or hemorrhaging RAM is laughably retarded and has zero basis in reality. Seriously people, find something that is at least partially true to complain about.
I stopped using Firefox somewhere around the second major version, and switched to Opera. Been an Opera-user ever since. Tried to go back to Firefox a few times, but I have too many complaints with memory-usage and loading times. Opera has a way more flexible interface, which allows you to re-arrange and customize the look and feel of everything. FF only allows you to do so much out-of-the-box, and many efforts would require that you script your own extensions. Speaking of extensions, Opera supports UserJS, which suffices in place of FF's extensions. I've been able to find the most essential scripts I've needed to expand Opera.
I can't stand Opera. It has some of the stupidest default settings I've ever used in a browser and no UI to change them. I'm not going to dig through it's equivalent of "about:config" to change something that should at *least* be in the options menu if not the default.
Outside of it's default behavior that I despise, though, it's a pretty solid browser.