If you come across this issue in your site, which can be viewed from the firebug console, probably the issue might be due to the conflict of Visual Editor with external plugins.
Just follow the below steps to fix the issues.
Kindly, disable all the external plugins, other than the theme recommended plugins and check the issue.
Please navigate to WP Dashboard > Users > Your Profile and check to see if the Visual Editor setting is checked – if it is, please uncheck it. Make sure you have logged in as administrator.