Web browsers do not process server side includes. That code needs to be processed by the web server or local server that sends the file and the resulting file should have a text/html content type.

Well, I understand the theory pretty well.

But when I ask Firefox to open a file, there is no server! So I figure that Firefox should recognize that and include processing for server side includes.

Or is Windows somehow the server here? That's not clear.

So I guess you just can't do it, eh? :-)

You can't use File > Open File to process server side includes. That will only work if you use File > Open Location and if there is a server that can process the file. If you use File > open File then you open that file directly in Firefox (bypassing a server) and only the HTML code will be processed and not SSI code.

http://encyclopedia.thefreedictionary.com/Server+Side+Include

All I'm saying, then, is that is a feature that could be added to Firefox, along with an option to turn the feature on or off at user specification. It should not be hard for Firefox, on opening a File, to notice that the name ends in .shtml and thus to process any SSI code it finds. That way one could test the appearance of rendered HTML before putting it up on the real server.