"TinyMCE remove-base-64-image" immediately removes images dropped into the TinyMCE editor with Firefox and alerts the user with the reason.
"TinyMCE remove-base-64-image" can only remove an image and alert the user after browser conversion of the image to base64 code has completed, and this takes longer for large images.
Make sure you have the following hook in your theme's header.php file just before
<?php wp_head(); ?>
Only TinyMCE, the default WordPress editor.