My firefox uses too much memory (Around 1.8 GM) when started even without any tabs open. I disabled AdBlock Plus addon and then the memory reduced to around 600 MB. So in my case, AdBlock Plus is the villain. Maybe i have too much custom filters in AdBlock Plus. Let me now check again after removing the custom filters.
My System Details.
Microsoft Windows Vista Home Premium 32 Bit. Service Pack 2.
3 GB RAM.