The other day I right-clicked on a word in Firefox and selected “Add to Dictionary” by accident. Here is how to rectify the mistake by removing the added entry.
1. Close Firefox
2. Open Nautilus to your Home Folder and click on View > Show Hidden Files
3. Navigate to: .mozilla/firefox/xxxxxx.default
4. Open the file called persdict.dat in the Text Editor and delete the unwanted entries
5. Save and Close the file