Google Chrome and IE8 have had multiple processes for tabs and extensions for quite sometime, however, Firefox has not yet come close enough to rolling this feature into the browser.
According to a blog post by Mozilla developer Benjamin Smedberg, the latest nightly builds of Firefox have support for Out-of-process plugins (OOPP). This means that Firefox will now load plugins like Flash and Windows media player as separate processes along with creating separate processes for tabs and windows.
This is definitely good news as if Firefox hangs while watching a video you can simply kill the mozilla-runtime.exe process to recover the browser, without having to kill the entire browser. In my tests, the browser did hang a few times, and killing the mozilla-runtime process allowed me to access it again. However, the nightly version is still a bit buggy as some weird problems cropped up during my tests.
However, this is definitely good progress and hopefully a stable build with Multi-process plugins and tabs should be available in next few weeks. If you are looking to try out and help testing the new Multi-process feature in Firefox, you can download nightly builds at http://nightly.mozilla.org. The multi-process feature is not available for Mac users.
Please note that the nightly build should only be used for testing, do not replace it as your main browser.