This option will force TinyMCE to load script using a DOM insert method instead of document.write on Gecko browsers. Since this results in asynchronous script loading a build in synchronized will ensure that themes, plugins and language packs files are loaded in the correct order. This will on the otherhand make the initialization procedure of TinyMCE a bit slower thats why this isn't the default behavior. So this option is set to true by default if the document content type is application/xhtml+xml.
Example of usage of the strict_loading_mode option:
tinyMCE.init({
...
strict_loading_mode : true
});