Mozilla has nerfed the addon api and addons are not able to do as much anymore as in the past.

As for the custom download folder, that's an anti-feature. The browser has a downloads folder so all browser downloads will and should go to that folder. My addon can download images to a subfolder of it, but that feature is not available in firefox yet because the api is not complete yet. It will be possible to use subfolders again at some point in the future, but the browser downloads folder will always be the starting point - it's a matter of technical complexity, reliability, cross-platform operation, and sane design.

What happened? Firefox became a completely different browser under the hood, that's what happened. And a lot of power was taken from addons, a compromise that had to be made to ensure quality in future addons. Don't like it? Switch to chrome, that's what I did.

Edit: yeah, you can roll back to a previous version and switch off auto updates on a per-addon basis. I wouldn't wait too long to update though because starting with firefox 57 the older versions will simply not work anymore.