Firefox offers no obvious way to clean single misspelled words or urls from the database, it looks like it is always delete all or none. There is however a simple way to get rid of those misspelled words easily. It is in fact that simple that I find it astonishing that many users have problems figuring it out.

All you need to do is to make the misspelled text popup, use your cursor keys to navigate to it and hit the delete button.