How to enable spell checking in Firefox? Enable built-in functionality
Type in address bar about:config
Then in next window type layout.spellcheckDefault
When option appear below double click on it and change value to be 2
Install any dictionary add-on for firefox you like from https://addons.mozilla.org/en-US/firefox/browse/type:3
Restart browser
To make sure it works right click on textfield and enable Check Spelling
Done 
