TA Z-Mail Doesn't Show Text —Blank !

I'm having a problem with TA Z-Mail userscript —it loaded the posts upon the first run & displays notifications but when I select or compose a post in it —it doesn't display anything —the content\ editor is blank ! I've used Z-Mail & it probably works on Chrome, but on Opera (before) & on Firefox Quantum it doesn't display text, weird~ I've emptied cache\ local storage, but still no difference. Any ideas ? I use Greasemonkey btw, & Tampermonkey before.