In Internet Explorer 11 and 10 on Windows 7 I noticed that NONE of the wysiwig editors in the WordPress admin display in IE. Plus, the textarea field never gets initiated and gets locked in a state that is not usable. I disabled all plugins and used the default twenty-fifteen theme with just the Clear-Floats-Button plugin and the issue remains. When I disable this plugin, the issue goes away.

I applied the exact same un-minified version of tinymce to trace the issue and the result is attached.

I don't have the time to research or provide a suitable fix BUT I am able to implement a workaround by commenting out line #64 of clear-floats-button.php

With the "custom_elements" line commented out the wysiwig editors work fine. I can add, remove Clear Float buttons w/o issue and the result on the front-end is the same. Which lends me to think why is the "custom_elements" option even needed? Might not be suitable for everyone but again, this hack works for me for the time being. Hopefully you can provide a proper fix. Thanks.