From Adblock Plus:
URL Fixer corrects typos in URLs that you enter in the address bar. For example, if you type google.con, it will correct it to google.com (asking first, if you enable confirmation).
This version will correct common misspellings of .com, .net, .org, .edu, .gov, .mil, and all other mainstream TLDs, as well as the protocol (http:, https:). By right-clicking on the address bar, you can set it to auto-correct your errors, or you can have it ask you before making any corrections.
You can specify your own set of custom corrections. Add or remove your own corrections in URL Fixer's options dialog, accessible via Tools -> Add-ons -> URL Fixer -> Preferences.
You can also correct errors as they happen; if Firefox can't find the website you typed in, URL Fixer lets you mark that URL as a typo, and you'll never see that error page again.