When Google started building Chrome, it made the decision to start from scratch. While this meant that there were a lot of things to do, it also meant that the developers had the freedom to make everything the way they envisioned it from the start, something that Firefox developers, which is built on Mozilla's legacy, itself based on Netscape, didn’t benefit from. One of the biggest decisions was making individual tabs and extensions run in their own process to insulate them from each other for security and stability purposes. This is one of Chrome's big selling point, but Firefox is catching up having enabled experimental multi-process plugins support in the latest nightly builds.
“Yesterday I landed multi-process plugin support in mozilla-central. By default, this capability is disabled, because there are still some serious bugs. But if you are willing to suffer some temporary instability, we could really use some help testing Minefield nightlies with out-of-process plugins (OOPP),” Mozilla's Benjamin Smedbergs wrote.
The feature is very early on in the development process and is limited in its scope for the moment. Currently, it works on Windows and Linux and the multi-process capability is limited to plugins, i.e. Adobe Flash and the likes. It's not particularly stable either, but if you're running the latest Firefox nightly build, this most likely isn't a major concern.
To enable it, type “about:config” in the address bar and find the “dom.ipc.plugins.enabled” variable. Set it to 'true', restart the browser so that the new settings take effect and hopefully everything should be working properly with plugins relegated to a separate “mozilla-runtime.exe” process (on Windows).
The feature is hardly ready for the prime time, so the best you can hope to achieve is helping the developers with bug reports when it eventually crashes. Interestingly, it uses quite a bit of the Chromium code built for the same purpose, possible as both browsers are open-source projects. The plan is to have it built into the browser by the time Firefox 4 arrives about a year from now, by which point it should split tabs and other components to their own processes as well.
Multi-Process Plugins Arrive in Firefox
Courtesy of Google Chrome source code