In Stallman's dream world, you would be free to charge for your service, or put ads up, or whatever you like. But your service would use standardized file formats, and the user would be able to modify the application (both server-side and client-side) to suit his needs.
I have zero experience with Firefox extension development, but everything I _have_ seen about it points at it being very difficult to mirror in a closed source environment.
- firefox - open, free, gets money from google
- openmoko - open mobile, you can even download their pcb design and CAD files, but they do sell the product
- specialised apps - company I work for releases some code as open-source, but that doesn't mean that our users have resources to provide the same service themselves, so we don't lose anything
edit: Sure, downvote me if you like. If you like to think adblock etc actually block all adverts I guess it's up to you. It blocks a small class of 'obvious adverts'. Several websites will pass outgoing links through a jump script which may or may not then go through affiliate links. Adblock is useless against such things. There are several such examples.
I don't do it to stop site owners 'making' money from me. If you can get non-annoying ads to me, and make money from me, that's brilliant. If that's all anyone did, I'd get rid of *block altogether.
Although I think what Stallman would probably prefer is the source to the site / web application being available so that people can run their own instances.
HN and reddit have their source available. I don't see anyone using the reddit code in a way that's any threat to reddit, mainly since the size of the community is an asset that's hard to compete with.
Potentially off-topic, since I'm not familiar with the prevalent licensing, but the various django applications look pretty nice for this kind of thing.