tinymce-docs-4x/_includes/configuration/gecko-spellcheck.md

gecko_spellcheck

This option allows you to disable Gecko based browsers' internal spellchecker from being used in TinyMCE.

Type: Boolean

Default Value: true

Possible Values: true, false

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  gecko_spellcheck: false
});