IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
Ironically, the easiest way is probably going to be by using yet another plug-in, about:addons-memory.
Here's what Softpedia says about how to determine how much memory your Firefox plug-ins are using:
"In order to determine how much memory each add-on uses, you might find about:addons-memory pretty useful. It is an add-on, so it does require RAM to function, but it includes it in the statistics and, from what we’ve seen, it’s less than 1MB.
"Restarting the web browser is not required after installation and you can put its name in the address bar to see the details. The data is presented both in raw figures and in percentage and there is also a graphical bar to compare the memory used by each entry."