Not working for me on Win 7 x64 SP1 with Firefox v54. I did have Text Area Cache installed but disabled but have removed that which has changed nothing. The toolbar button shows "There is no cache here!" when it is clicked. That is the same with this Mozilla extension edit box.
The reason why it kept telling you 'there is no cache' even if you give some text to textarea is because addons.mozilla.org do not allow any add-on work in this page. You could try type something in textarea in other sites, if it keep telling you there's nothing, please give me the url, I will test it, and fix bugs for you (if there is any)!
