How to remove words that you accidentally added to the Firefox dictionary:
- Open the Firefox menu.
- Open the Help menu (question mark).
- Click on Troubleshooting Information.
- Under Application Basics, find Profile Folder and click Show Folder.
- Locate the file called persdict.dat.
- Right-click on the file and open it with a text editor. There might be an option to Edit with [name of text editor] in the right-click menu.
- Find the misspelled word and delete it.
- You can also add new words here, with each word on a new line.
Video:
https://www.youtube.com/watch?v=Xssh8e52_Rs