Just Download It Already by leere

Fork of
https://addons.mozilla.org/en-US/firefox/addon/save-to-folder
https://github.com/edjroot/webext-save-to-folder
with small addition - if you're using only one defined folder, then drop-down menu will not to be shown, instead it will be one-click download straight from main context menu.

After installation In the extension's settings page, set up a list of folders. These will appear on your context menu so you can download stuff directly to them.

There is plenty similar extension like "Save Image to Downloads", this different by - it is also support download media files (like mp4, webm)