In Mozilla Firefox type into the address bar:
about:memory
Now you're presented with a full page of memory related information in Firefox.
One neat thing is the 'Minimize memory usage' button located at the bottom of the page.
I think this has been implemented for a few versions, but I'm not sure because I primarily run Nightly which is 3 versions ahead of the release channel.
Possibly also of interest: https://addons.mozilla.org/en-us/firefox/addon/about-addons-memory/