While Silverlight promises a future where rich client applications written in .NET work seamlessly in all web browsers, many developers cannot afford to wait. And that means using proven deployment technology that is already available.
Currently there are three ways to deploy rich client .NET applications. The simplest is to just offer the executable, possibly wrapped in a zip file with its dependencies. Unfortunately, this does not lead to a very rich user experience. Installation activities like creating shortcuts do not occur, and automatic updates are not even a consideration.
The next option is to offer a MSI package. While this does support installation activities like COM registration and shortcuts, it also lacks automatic update support.
This is where ClickOnce comes into play. Once installed, ClickOnce offers automatic updates based on declarative rules and/or an API. Other features include the ability to partially install applications, downloading other components as needed, and the ability to launch applications from URLs. One can even pass startup parameters to the application in this manner.
Unfortunately, until recently ClickOnce was not supported on non-IE browsers. When it came out in 2005 this may have been acceptable, but with Firefox continuing to gain market share it is no longer appropriate to require users to run IE.
This is where James Dobson picks up the slack. Via an extension he is calling FFClickOnce, Firefox users can now install and launch ClickOnce applications without switching to IE.