I remixed about:memory using D3 to make a treemap visualization of how loaded URLs are using memory in Firefox.
Load it by clicking the widget on the add-on bar, or Cmd/Ctrl+Shift+Y
This visualization focuses on which web pages are using large amounts of memory - this is not a complete accounting of all memory being used. This doesn't yet show how much is being used by some of the Firefox internals.
Add-on installation does not require a restart. Only tested on Nightly builds.