Firefox has had a built in inline spell checker for a looong time now. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker Only a few language locales comes with a dictionary due to licensing reasons so you may need to install a dictionary from https://addons.mozilla.org/firefox/language-tools/ (ignore the language packs as they are for changing the language of the UI like menus) There are a number of Grammar related extensions at https://addons.mozilla.org/firefox/search/?q=grammar