The guys at Mozilla have been trying their hands at multiprocess Firefox. The feature has been available in Google Chrome for a long time now.
A success on this was achieved by borrowing open source code from the Chromium project which allows multiprocess Chrome tabs. The advantages of multiprocess are that with multiprocess tabs, we can have the application and the rest of the tabs safely running, even if one of the tabs crash.
In a blog post, Benjamin Smedbergs says:
Currently only Windows and Linux support multi-process plugins: mac support requires additional work. To turn OOPP on, visit about:config, find the pref dom.ipc.plugins.enabled, set it to true, and restart your browser. Please report any crashes or instability in bugzilla: product Corecomponent Plug-Ins.
You can see the current status of the borrowed code here.
The feature is available in the latest nightly build and is disabled by default. To enable the feature, go to the address bar and type in about:config. Look for the key value dom.ipc.plugins.enabled and toggle its value to true by double-clicking on it. Restart for the change to take effect. Now, you should see your plugins, running in a separate process each.