According to Ryan Paul at Ars, the new feature has just landed in a developer preview build and is the first real world work from the ambitious Electrolysis project. I have been following this effort by Mozilla to bring true multiple process support to Firefox since its planning stages. Ryan provides some excellent detail in the article, in particular noting that the crash protection is limited by each plugin running in its own process.
The example he provides is multiples browser tabs with many embedded Flash and Silverlight objects. When one Flash object crashes, they all crash even though the Silverlight objects and the surrounding content remain unaffected. This actually seems like a decent compromise as using a process for each instance of a plugin could get pretty demanding of the underlying operating system.
Ryan also points out that Mozilla is looking for testers of this build, so if you don’t mind working with a bleeding edge version, check out the link.