MemBench

This benchmark automatically opens 150 sites with a 1.5 seconds pause after each new site. A textbox appears after 90 seconds indicating that the benchmark has finished.
You can see detailed information about your memory behavior if you type about:memory in your Firefox URL bar.

Firefox users have to disable the popup blocker for my site and adjust dom.popup_maximum to 200 in about:config. If you want to monitor the memory behavior after closing all sites, also close this site.
Chrome users have to run this script.