Update: URL Fixer was acquired and is now hosted at http://urlfixer.org/
URL Fixer was inspired in 2006 by this Firefox bug report. Since then, it has been a featured add-on on the Mozilla Add-ons Gallery, it was one of the first add-ons to be compatible with Mobile Firefox, and it placed in the Extend Firefox 2 contest. It used to be compatible with both SeaMonkey and Flock (remember Flock?); its functionality was included in Netscape Navigator 9, and it was at one point under consideration to be included in Firefox 3.
URL Fixer has also been the subject of several experiments: it was the source of the statistics I used in my examination of what people type in the address bar, and it was the add-on I used to test the feasibility of selling a freemium browser add-on.
The new team in charge of URL Fixer recently released version 4, which you can install without needing to restart Firefox; I’m looking forward to seeing what other improvements they make and in what direction they take the add-on. Please note: support questions should no longer go to me; please send them to firstname.lastname@example.org.