Since a few months I was looking for a way to force my web browser to redirect me from http to https protocol for some websites I often use if I forgot to type the correct url. The best solution was of course to develop a Firefox plugin, but I really don't have time for this. I finally found a way with Greasemonkey. It's a Firefox plugin which allow you to execute javascript code for websites you decide; perfect. You will find the very small file in attachment.

I agree that's not a very clever solution, because the script is starting after (and not before!) the web page is loaded. So the page is loaded, and if the the rule matches, it rewrites the url with the correct url; a new request is launch.
So it's useful if you are going to do some other stuff on the website and not reading the first page loaded. The aim of such a Firefox plugin would be to detect theses url before launching the first request to the server, and so rewrite the url before this.