There seems to be some new default behaviour change from Firefox after a new update of the browser, Pls. follow the guide here - https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file to configure auto-save/auto-play of media files.
You can find the download folder using the instructions below. If you are manually picking file-save folder location every time a download starts you can find the files by :
(a). Looking in the Download Manager window, Use Ctrl + J to open it.
(b). Start a new download and see where it saves the file.
Easy Youtube Video Downloader uses your web-browsers default download folder location to save downloaded video/audio files. You can change/find this location using the instructions for your particular web-browser as shared below.
Find or change Download Folder Location and stop autosave For Mozilla Firefox >>
Find or change Download Folder Location For Google Chrome >>
Find or change Download Folder Location For Opera >>
Note : If you want your downloaded videos and audio files to appear in your media library, follow instructions provided here - http://youtube-downloader.uservoice.com/knowledgebase/articles/261068