You can already do this.
Internet Explorer's context menu extension mechanism has been in MSDN for years. Let me show you how you can create this extension yourself.
First, create the following registry key:
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\MenuExt\View in Mozilla] @=REG_SZ:"C:\some\path\to\ViewInMozilla.htm" Contexts=REG_DWORD:1
Of course, you need to change C:\some\path\to to an actual path.
How did I know to do this? Because steps 1, 2 and 3 in the "Implementation Steps" section tell me (1) what key to create, (2) what to set the default value to, and (3) what to set Contexts to. I chose a Context value of 1, which means "Default".
<SCRIPT> alert(external.menuArguments); </SCRIPT>
Okay, let's run this puppy. Launch IE, right-click on a blank space in the web page, select "View in Mozilla", and you get...
Woo-hoo! This is a major accomplishment: Something happened at all. Doing things in small steps makes it easy to identify where a problem is. If we had run full steam ahead to completion and then it didn't work, we wouldn't have known whether it was due to a bug in the script, a bad registration, a bad filename...
Now that I have the menu arguments, I can use that to suck out information about the item that the context menu applies to. Let's try this:
<SCRIPT> alert(external.menuArguments.document.URL); </SCRIPT>
Woo-hoo, now it gives me the URL. Almost there. All that's left to do is to run a program with that URL as the command line parameter.
<SCRIPT> var shell = new ActiveXObject("WScript.Shell"); shell.run("mozilla \"" + external.menuArguments.document.URL + "\""); </SCRIPT>
Now you too can create Internet Explorer context menu extensions.
In fact, go ahead and do it, since Darrell asked for it: Create an Internet Explorer context menu extension that operates on anchors and opens the linked-to page in Mozilla.