Perfect, except... Rated 4 out of 5 stars
It would be nice to add custom rules, but this extension is great. It's light, an easy install, and a great addition to any Firefox pack.
I RECOMMEND THIS ADDON -- download and try it! :-)
I've developed a premium version that allows you to add custom correction rules: http://www.chrisfinke.com/marketplace/url-fixer-plus.php