From MozillaZine Knowledge Base

persdict.dat stores custom entries added to the spell checker dictionary.

Editing

There is no issue editing this file as long as Firefox, Thunderbird, or Mozilla Suite/SeaMonkey is closed. The file's format is one custom dictionary entry per line.

Moving

This file can be moved to a different profile.

Deleting

Deleting persdict.dat will delete all custom entries in the spell checker dictionary.

See also

• Deleting spellcheck entries
• Spell checking