I accidentally added a word to my spell checker dictionary on Mozilla Firefox, how can I fix this?

I cannot find any options to get to the spell checker. I need to be able to delete the improper spelling of that word. I never meant to add the word, but was using my laptop while sending a message to someone. When the spell checker questioned that word, I was using the touch pad, and attempting to scroll up to select the proper spelling of that word.. unfortunately it was hovering over the "add this word" option and took that to be my selection. I should have canceled the spell checker but panicked... and sent the message anyways. Now each time I misspell that particular word in the same way, it will not be flagged.I accidentally added a word to my spell checker dictionary on Mozilla Firefox, how can I fix this?
Just start misspell another word, then when that pop-up box comes up, there should be a scroll down box with all of your added words in it, select the word that you misspelled, then hit the delete key.