Firefox 3.6.4 has just been released by the developers after a longer than usual wait time. The new version of the web browser comes with out-of-process plugins enabled. This feature basically runs plugins in their own process to increase the stability and security of the browser.
By default, only Silverlight, Flash and Quicktime are making use of this feature. Every other plugin is still running in the main Firefox process.
The developers have however created an option to add other plugins to the out-of-process plugins list so that they are run in their own process as well….

How 2 with screenshots.

Source: Run Custom Firefox Plugins In Their Own Process
http://www.ghacks.net/2010/06/24/run-custom-firefox-plugins-in-their-own-process/