Most of the functionality needed to do this is built in.
To view resource usage of pages
about:memory. This gives you an interactive tree of memory allocations (click on an item to see more details; mouse over to see a detailed explanation of what each entry means):
explicit/layout you will see the memory used directly to display each tab.
http://superuser.com) rather than page. These numbers represent how much memory scripts running on those pages use in total.
To view resource usage of plugins
use your operating system's process viewer/system monitor/task manager. Firefox now runs plugins in separate processes, so by looking at each process's memory/CPU utilization you know the performance of the corresponding plugin.
On Linux, the plugin processes are called
plugin-container. Look at the command line of each process to see which plugin it represents.
When a plugin crashes (or you kill its process), you'll see a message on the open page(s) containing instances of the crashed plugin.