English (US) Dictionary от flod

This add-on repackages the en-US dictionary available in mozilla-central, which uses the normal word list by SCOWL — size 60, see their README for more details — and integrates it with over 6 thousand words.

For information on how to use dictionaries in Firefox, see this support article.

To report issues with the spell checking feature in Firefox, please report a bug here.

If you want to add or remove words from the dictionary, follow the process described here.