USE: Go to "about:addons-memory" (without the quotes) in your URL bar. Here you can see the memory usage of addons. Note: These are not meant to be used to show you if your addons are"bad" addons, but to show you how much memory any addons are using period. Some just use a lot.
NOTE: Currently working with Firefox versions up to 48.x
Betas, Nightlys and Alphas are UNLIKELY to work.
Enjoy and please check out SturdiGuns.com! Thanks!
For more information, check out the original addon's page.