about:addons-memory provides an experimental about: page for advanced users to display some memory usage statistics about their add-ons.
To use, open about:addons-memory in a new tab.
no, it is not OK to install this add-on along with the Top 20 or something add-ons and publish articles singling out the "biggest memory wasters" or something along the lines of this. This is wrong, superficial, sensationalist and outright wrong.
Adblock Plus, being the most popular add-on, using 20-30 MB of memory is not a problem. Add-ons that may use hundreds of mega bytes of memory with steadily growing memory usage *may* be a problem and this add-on is intended to find exactly these kinds of issues more easily.