Firefox 4.0 will include a feature that runs the browser in multiple processes, like Chrome:

Electrolysis is the name of a Mozilla project that will split Firefox into multiple processes — one for the user interface, one for plugins and one for each tab. Similar functionality is already being seen in other clients, like Google Chrome. Like Chrome, a crash in a single tab will no longer be able to bring down the entire browser. Also, we can expect Electrolysis to make Firefox faster and more stable overall.