Have you ever accidentally made a typo when filling in your username or email address in a login form? Has Firefox remembered that one mistake forever? Does it annoy you to see that badly spelled version of your name pop up in the autofill suggestions every time you try to login?
Fear not! The solution is ridiculously simple.
Start typing your username / email address / whatever in the input box and wait for the autocomplete options to pop up. Use the cursor keys to highlight the option you want to delete. Hold down shift and press delete. Simple as that.