If you’re a Firefox user, about:addons-memory will help you find the add-ons that are dragging your browser’s performance down.
about:addons-memory doesn’t need the browser to be restarted upon installation. It adds a new about page, about:addons-memory that gives you a summary of the memory consumed by all installed and enabled add-ons in Firefox. Memory usage is shown in MB, followed by an Add-on column that shows the add-on’s relative memory usage in comparison with the others enabled add-ons. A ‘Minimize memory usage’ button at the bottom of the page allows you to reduce the memory being used, but it is a temporary solution. Memory usage might also fluctuate depending on whether an add-on is actively working currently or not.
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.
It does appear to work ok on firefox 16, 17 and 18.