How to Configure Firefox's Built-In Spell Checker and Custom Dictionaries

Firefox includes a built-in spell checker that underlines misspelled words in text fields, comment boxes, and editable web areas, offering corrections and options to add words to a personal dictionary. Users can toggle spell checking per field through the right-click menu or globally via Settings or the about:config setting layout.spellcheckDefault. Additional language dictionaries can be installed from addons.mozilla.org or the Manage dictionaries option, and custom Hunspell .dic and .aff files can be placed in the profile's dictionaries folder. The personal dictionary, usually stored as persdict.dat, can be edited directly to add domain-specific terms, and spell checking can be disabled for coding contexts. The built-in checker covers spelling only, does not check grammar, and skips password fields, the address bar, and inputs that sites opt out of.