I'd like to know when and how the firefox 4 plugins eats memory.
At least with linux you can look for a process called plugin-containe. I assume its the same with windows.
Under linux you can use:
ps v -C plugin-containe
That will print out memory information about the process. That information refers to all plugins running in FF, because all run in the same process. I don't know if there is still any memory allocated in the main FF process related to plugins (e.g. some IPC overhead).
One of the printed information is a column with the percentage of your physical memory.
%mem %MEM ratio of the process's resident set size to the physical memory on the machine, expressed as a percentage. (alias pmem).