As you already know, current Firefox version (2.0.x) consumes too much memory (even when it's being idle). Several improvements have been designed to reduce this memory usage and it will be available when Firefox 3.0 shipped. Meanwhile, for you who wanted to optimize Firefox's performance, you can try this tools : Firefox Ultimate Optimizer (via Antonius Rachmat). It's a tool which can work silently to reduce Firefox's memory usage, thus frees your system with heavy memory loads.
Too bad, it only available in Windows platform and it requires .NET 2.0 to be installed I boot to Linux more often than Windows, so i can't use this application. It's very nice application though.
Currently, this application doesn't have any configuration yet, so you can't tweak anything. One more weakness about this application is that the icon can't be hidden, so it will be on your task bar all the time. Future version might solve this, but i'm not sure yet (i'm not the developer).