This is one thing I thought would be quite straightforward to do, but bizarrely for Firefox it is a bit obscure. I love having the spellcheck facility and the ability to add words to your personal dictionary. The first time I realised I'd accidentally put some gobbledegook in there, I was stumped for a while as to how you edit it.
Anyway, I discovered what you have to do is open windows explorer and navigate to the folder:
C:\Documents and Settings\{user name}\Application Data\Mozilla\Firefox\Profiles\4jotvncu.default
look for the file called persdict.dat and edit it in notepad or wordpad.