It is theoretically possible. I mean, a Web browser is just the application. It can do whatever you want
if you design it to do so, so it could communicate with some desktop applications, too.
One way to do it is this: you can create your own .NET application embedding
System.Windows.Controls.WebBrowser (WPF) or
System.Windows.Forms.WebBrowser (
System.Windows.Forms), please see:
http://msdn.microsoft.com/en-us/library/system.windows.controls.webbrowser.aspx[
^],
http://msdn.microsoft.com/en-us/library/system.windows.forms.webbrowser.aspx[
^].
Also, on Windows, IE officially allows to use an ActiveX object on the page. This is another possibility to "leak" access to the desktop world from the Web application. (Some other browsers allow it unofficially, in the form of browser plug-in.)
Please see:
http://en.wikipedia.org/wiki/ActiveX[
^];
this is how:
http://msdn.microsoft.com/en-us/library/ie/ms970419.aspx[
^].
This article explains why this is a
very dangerous thing:
http://en.wikipedia.org/wiki/Drive-by_download[
^].
How about other "official" ways to access desktop machine from the Web applications by a reliable well-known browsers? The answer is: they don't do it, because if they did, they could not be trusted — this would be a
serious security hole. I personally don't trust even the latest version of IE to browse non-trusted or unknown sites; I still need convincing arguments to trust.
[EDIT]
As to development of Firefox extensions, I hope you can trust yourself if you do it for access to your local machine. I'm less sure about your users. :-)
You can see this manual:
http://www.rietta.com/firefox/Tutorial/overview.html[
^].
See original documentation:
https://developer.mozilla.org/en/Extensions[
^],
https://developer.mozilla.org/en/Building_an_Extension[
^],
https://addons.mozilla.org/en-US/firefox/addon/web-developer/[
^].
—SA
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