IF NO DOWNLOAD BUTTON IS VISIBLE:
A). Try to open a video in PRIVATE MODE (Firefox Menu->New Private Window) to make sure you are not seeing an earlier cached version of the webpage.
B). The download button could be also missing due to Youtube's experimental layout issue, Youtube tests variations of their user-interface with selected few random users every now and then. The button shows-up on the old interface and you might have some new experimental interface, click the Exit icon in lower right corner of youtube page to get back the old interface. On some new tests you need to click the "three dots" icon before SIGN-IN link and click "Restore Classic Youtube", you should get back the old layout and the download button.
C). If any of the above not reinstall the addon from here - https://www.yourvideofile.org/
Feel free to get in touch via Support form if you are having problems getting the download working and we will help you out.