About:addons-memory is a simple Firefox extension which tells you how much RAM your various addons are using.
If you want to explore Firefox's memory use then you could always enter about:memory into a new tab, but the report you'll see is highly technical, with details about heaps, compartments and so on. This extension is far more straightforward.
Installation is quick and easy (there's no restart required). Once it's done, open a new tab, type about:addons-memory in the address bar and press Enter for the details.
What you'll then see is a list of extensions sorted according to their RAM use. The "Memory" figure tells you how much RAM each addon is using; "Addons" gives this figure as a percentage of the total memory used by your extensions, and "Explicit" gives the figure as a percentage of Firefox's memory use.
Scroll down to the bottom of the list and you'll find a "Minimize memory usage" button, which may be able to free up a little RAM, while a couple of notes explain more about the report.
Version 12 brings "compatibility fixes" (Full Version History).
A simple way to find out more about Firefox memory usage