I was expecting firefox 4 will support process separation between each tab as Google Chrome does. But it is not the case for version 4.
However the good news is that from version 3.6.4 it has a protection in form of separate process for some plugins called plugin-container process. So the crash of plugin won’t cause whole browser to crash.
You can find more info here: http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins
30/10/2011: An update:
As explained here – http://arstechnica.com/civis/viewtopic.php?f=14&t=1155803 Firefox can be built with Electrolysis flag and then it supports this feature. But it is still at early stage and the earliest FF version where we can have it is Firefox 10 comming in February 2012. I’m looking forward to it…