I would use Firefox itself to do this. I remember that Netscape was somewhat controllable from the command line - you could tell the currently running instance to load a certain page, save a page, etc. If Firefox is controllable in the same fashion I would look into that kind of solution.

Otherwise you can probably do what you want to do with javascript if you can bypass the "same origin policy". Just write a page that loads your target page into another window and write the javascript you need to emulate button clicks, text entry, etc.

Finally, you might be able to find a Firefox plug-in which does this, or you can write your own plug-in.