The extension Smart Find for Firefox browser helps in such situation. This extension gives an improvement to the Find method in Firefox, which allows a user to find a word by just giving clues.
After installing the extension, when you press Ctrl + F to find a word, a check box labeled "Similar" will appears in the Find-Bar. Check this box, if not and write a word, if it doesn't match with any term, press ENTER button, the SmartFind method will run to find the most similar word in the current page.
And also, in the file menu click Edit - Find Similar Level, and select Low/Medium or High from the list. These levels indicate how similar the found word is to the original word. Install SmartFind.