SmartSpell by John

Smart Spell replaces basic dictionary-only spell checking with faster, context-aware spelling correction directly inside Firefox.

Features:

Red wavy underlines for likely misspellings
Right-click correction suggestions
Context-aware ranking using word and bigram frequency data
Works in text fields and contenteditable editors across websites
Add words to a personal Smart Dictionary
Runs entirely locally
No typed text is sent to Google or any other external service
No analytics, tracking, advertising, or account required

Smart Spell uses English frequency data derived from the SymSpell project. The included frequency dictionary combines Google Books Ngram frequency information with SCOWL English vocabulary data.

Smart Spell is currently designed for desktop Firefox.