Global extensions are extensions that are not installed into a profile. Therefore, the extensions is always available, even in a new profile. Yes, I am talking about extensions, not plugins. Plugins can also be global, there are a few ways to achieve this on the various platforms (Windows, Mac and Linux).
However, only Windows had the capability to install global extensions, using the Windows registry. Thanks to bug 311008 and bug 412449, global extensions are now possible on Linux and Mac too. The magic paths:
(or /usr/lib64/mozilla/extensions/ ) /usr/share/mozilla/extensions/ ~/.mozilla/extensions/
Note that it is possible to limit the extensions to a specific user as well. The new MDC documentation goes into more detail. Hopefully, this makes deploying 3rd party extensions a bit easier.