I am developing a web application. I would like to extend my error messages (and their backtraces) so that I can click on the mentioned file, and have it opened automaticall
Pekka,
After reading the thread so far, it seems that you want to build an application that somehow authenticates with the server--i.e.: the "average user" wouldn't have access to it. If this were the case, then delivering it through the browser is an impossibility without writing a custom extension.
Running authentication through GreaseMonkey is difficult, but once the client is authenticated, there is no real way to "run" the trace.
If the server generates a batch file or some kind of instruction set (script, shortcut, etc.), you can simply configure the browser (or have the local instance of your app configure the browser) to run the file. The problem here is that you have no way to automatically authenticate!
The only other way I can imagine that you could get this to work is via a Java applet, which would only be cumbersome and require Java to initialize every time you wanted to import a trace.
The problem you have is that the browser is inherently secure. It's designed to protect the computer from malware, rogue websites, etc etc., and so without developing a custom extension for the browser, there's no way to make the hop to any applications that run in tandem with the browser.
So on that note, I'll suggest that you reconsider writing a Firefox XUL extension as mentioned above. You'll probably need to implement some XPCOM code to make it work, too. Here are some resources that will help get you started:
https://developer.mozilla.org/en/xpcom
https://developer.mozilla.org/En/XUL
http://ted.mielczarek.org/code/mozilla/extensiondev/
https://developer.mozilla.org/en/XUL_Tutorial/Introduction