In this post you will find an easy to implement solution to the problem of non-working spell checker in fields for entering e.g. product descriptions in PrestaShop 1.6 stores.
This solution should enable spell checking in all browsers. Remember to enable spell check in your browser's settings.
To make the change, find the tinymce.inc.js file in the jsadmintinymce.inc.js directory
In the file we find the line: browser_spellcheck : true,
And add under it, in the next line, the following code: gecko_spellcheck : true,
After saving the file, it remains to check if highlighting errors in the field to enter a description works.