Somebody complained that they installed this extension but then found no way to use it. I had the same problem - at first. The icon that's supposed to appear in the toolbar didn't. But it turned out I had the same problem with other extensions. Firefox was somehow stuck in a state where no extensions worked. I found the problem described in several places. I tried 2 or 3 suggested solutions but none of them worked. In the end I uninstalled and reinstalled firefox. That did it. Cookies.txt now does the job.