Note that this should fix the main issue, but even with this fix the editor hasn't been fully tested in Edge so there might be other problems that we're not aware of at the moment...
Themes/default/scripts/editor.js
Find
oMyStyle.styleSheet.cssText = document.styleSheets['editor_ie_css'] ? document.styleSheets['editor_ie_css'].cssText : document.styleSheets['editor_css'].cssText;
Replace
if (oMyStyle.styleSheet)
oMyStyle.styleSheet.cssText = document.styleSheets['editor_ie_css'] ? document.styleSheets['editor_ie_css'].cssText : document.styleSheets['editor_css'].cssText;
else
oMyStyle.sheet.cssText = document.styleSheets['editor_ie_css'] ? document.styleSheets['editor_ie_css'].cssText : document.styleSheets['editor_css'].cssText;