Is it really plausible? It is hard to tell and it's in the gray area where it depends on many other factors. ReadWriteWeb have published interesting article Can Browser Add-ons Be Businesses?
Conclusion is as follows:
Browser add-ons are increasingly interesting ways to reach consumers. Since the browser is the most used application on the desktop and major browsers are platforms, businesses are looking for opportunities to reach consumers through this new channel. Better than desktop applications, browser add-ons are light and update automatically.
As with any vertical business, only a few add-ons can become real businesses. The competition is tough and the business models have not been mapped out that well. Yet if there's a shot at reaching users via a download, browser add-ons seem to be it.