Plugin memory usage tracking is only realistically possible for Chrome where plugins run in a sandbox with a really limited API. Even then, if the plugin manages to allocate memory inside a page's sandbox (for example, by injecting some CSS or JS in there), the reporting will be wrong.

The adblock plus problem is in fact one of those - it injects styles into pages in order to filter ads, which increases the memory footprint of the page. So neither Firefox nor Chrome can tell you 'adblock plus is increasing memory usage by X', because the memory usage is hidden along with everything else owned by the page, not by the addon.