This simple Firefox extension can be used for automatically enabling the spellcheck="on" attribute in the class="CodeMirror-code" div text input area in GitHub CodeMirror text editors so that when page is loaded the browser will default to highlighting misspellings (and allow selecting from suggested correct spellings).

To install this extension for Firefox follow the guide found here: https://riptutorial.com/firefox-addon/example/26613/installing-a-temporary-add-on