Internet Explorer 8 was the very first web-browser to utilize separate processes followed by Google Chrome, the separation of UI and multiple “content” processes to host web-content results in better security and performance. Using separate content processes allows them to be properly sandboxed enhancing security and will also result in better UI responsiveness as the browser UI would not be affected by poor performance of content processes.
Although the underlying Gecko platform on which Firefox runs supports multiple processes, the Firefox frontend is not designed to use them. Work to make the frontend support multiple processes began in early 2013, and fruits of the work are now available in Nightly builds of the browser. The new multi-process Firefox will make it a more stable web-browser, a crash in “content” process will not kill the entire browser and will show something like below telling which tab crashed allowing them to be reloaded.
Users can get this latest nightly version of Firefox from http://nightly.mozilla.org/, but do keep in mind that this is recommended only for advanced users and might not be stable. To use the new multi-process feature, create a new profile (not necessary but highly recommended) and set the browser.tabs.remote preference to true (via about:config) followed by an browser restart.