Firefox is your nanny?
Extensions are cool
Extension frameworks are a good way to add new functionality to large applications that are messy to alter directly, they also help to quickly add functionality now, without having to wait six months for the next release.
The whole fun of extensions is that you can make your own and download random ones from the Internet, try them out for a bit, remove the ones you don't like, share the ones you do.
This of course implies two things. Firstly, that the extension interface is well thought out so that a badly written extension does not crash or unexpectedly interfere with the main application. Secondly, that the user who installs the application is adept enough to know what they are doing.
The killer feature of Firefox has been its extension framework, having hundreds or even thousands of useful extensions is what has enabled Firefox to break into the IE-dominated browser market where so many have failed.