I grabbed HTTPS everywhere web ext couple of days ago. DTA will not be re-written in Web extension.
I hope this doesn't mean you're going to stay on FF56 forever and ever. I'm sure there are other download managers on there, I just haven't checked any out just yet. I'm under the impression that, as one developer said, his extension was heavily dependent on how the old Firefox worked and would be difficult to rewrite.
I don't care how much people complain. The update to FF57 and it's new engine was
badly needed. Newer, better addons will be written, you'll see,