One of the things that I really like in regards to Google Chrome is the browser's task manager that reveals a variety of information about the browser's memory usage. Firefox's about:memory page may reveal similar information, but the way the information are displayed are not of real use to most users of Firefox due to the way they are displayed.
If you have installed a lot of add-ons for instance in Firefox, you can't really find out which add-on uses the most amount of memory, and which is as light as a feather in this regard. This is especially important if you are running dozens of add-ons and notice slowdowns or memory issues when you use the browser for an extended period of time.
About:addons-memory is a brand new Firefox add-on that reveals information about the memory usage of add-ons in Firefox to you. You can use the feature to audit all of your add-ons for instance, to find out which add-on uses the most memory and maybe look for an alternative or remove it completely from the browser. That is, unless it is an essential add-on that you can't do without.
To use the extension simply install it in Firefox and load the about:addons-memory page afterwards in Firefox. Here you see a list of all add-ons that you have installed in the browser as well as a listing for the browser itself.
Each add-on is listed with its name, developer, unique identifier, and memory usage. The extension furthermore lists the percentage of memory that a particular add-on uses in relation to all other add-ons.
The developer notes that add-ons may use more memory than shown here.
Many Add-ons, especially non-JetPack ones, may include static or dynamic overlays that may add scripts, DOM or styles directly to the browser. Such things cannot be distinguished from the browser itself and hence cannot be counted towards the add-on. As a result the actual add-on memory usage may be a lot higher than reported here!
A button to minimize memory usage is also available that you can click on to reduce the memory usage of the browser temporarily.
About:addons-memory is a useful albeit limited extension that provides you with information about the memory usage of all add-ons you have installed in Firefox. It is nothing that I'd run all the time in the browser though, more something that I'd run to assess the memory usage of all extensions installed in Firefox and then uninstall it again once that has been done.