This page can be used to initiate a memory profiling server that will allow you to inspect the state of Firefox's memory state. While the server is up, your Firefox instance will appear to be "locked up".
Once the server is up, you can visit it at http://127.0.0.1:8888 using another browser. To stop profiling and resume Firefox, visit http://127.0.0.1:8888/stop using another browser (or click on the disconnect button in the tool).