It might have happened to you as well that you have added a word by accident and want that word removed again from the dictionary. This is the case for example if you add a misspelled word to the dictionary file.

Firefox does not offer an obvious way to delete words from its dictionary which can be quite frustrating if you write the misspelled word regularly. All new dictionary words that you add are written in the file persdict.dat which is located in your Firefox profile folder. The same holds true for Thunderbird as well and probably for all other Gecko engine based applications.

I suggest you search for the file persdict.dat on your system, close Firefox before you proceed and open the file afterwards in a text editor that is able to save line breaks. Open Office Writer, Microsoft Word, Ultraedit and Wordpad are suitable programs for this.

Locate the misspelled word(s) and remove them from the dictionary file. It is also possible to add words to it, just in case you were wondering. Save the file afterwards, the misspelled words are now gone and won’t bother you anymore.

credits: FFExtensionGuru