Unfortunately I really think this monopolist might just be Microsoft again. The web operating system is the browser, and when Microsoft start shipping silverlight with IE...

The biggest challenge to this will be getting high performance out of javascript in firefox. Also a combination of svg for graphics and access to the local system via a google gears type api (I see firefox 3 has something like this planned or implemented). While this would all be nicely backwards compatible it wouldn't give you cut+paste support, some sort of MIME support for edit boxes/forms is needed perhaps.