This is very similar to an idea that Jef Raskin presented many years ago in his book, namely to create an operating system that allowed users to add commands. If I remember correctly, Jef envisaged these commands to be purchased rather than downloaded for free, not foreseeing that open source software would replace much of the commercial market of this kind. I remember wondering at the time how this was any different from Unix, where you can string together commands using pipes to gain additional functionality. From my recollection of reading the book, I don’t recall that Jef explained in detail why users should extend the command set themselves. Having used Firefox for a while now, I think I understand what was intended
I use about a dozen extensions in Firefox (and it remains stable, touch wood!), but if you’d put me in front of a machine that had all of these extensions available to start with, I would not have known what to do. This is why I can’t bear to use Opera: it has a lot of nice features, but they aren’t very accessible. If Opera were modular, I might like it better. So the message is to allow people to extend functionality themselves, because that way, they grow with the technology and can better adapt to it.
Web applications are often limited by current broadband speed and availability, as well as server response times; Firefox extensions are helping to fill this temporary lag.
Incidentally, has anyone tried Onspeed? Their proprietary compression technologies sound impressive, but I wonder whether they have the bandwidth and processing power to match their claims that they can speed up even 8Mb/s broadband.