The desktop Firefox web browser has had a inline spell checker built in since Firefox 2.0 released way back in Oct 2006. This kb article shows how to use the spell checker in Firefox https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker Note only a few language locales comes with a dictionary installed like say en-US so if you have no dictionary then you can install one at https://addons.mozilla.org/firefox/language-tools/ . Ignore the language packs on the page as they are for changing the language of the Firefox menus and such.