Fix Code Editor on Firefox

example link: https://connect.nuxeo.com/nuxeo/designer/#/fsantos-SANDBOX/layouts/search/search/form

Steps to reproduce the error:

• In Studio create a Page Provider
• Go to Designer
• At Layouts go to the created Page Provider
• In the properties catalog drag and drop the edit mode of the first element into the editor. Do the same with the other elements.
• In the Layout Editor switch the view to code and start to edit it

Note: Small video of the error attached below