Hi Andy:
If you previously installed the extension, first check to see if it's still listed. Click on the hamburger (three lines, top right), then on Add-ons. Then click on Extensions in the left-hand panel (if it's not already selected). See if Easy Youtube Video Downloader Express is listed and not shown as Disabled. (If it is shown as disabled then, obviously, click to enable it).
If the extension isn't shown, type its name into the box at the top right and hit enter. Then click Install next to the correct extension.
Once you've got the extension installed, whenever you go to a Youtube page there should be a big, green 'Download As' button underneath the video. Click on that to see the options. Most of them will work without payment but the HD video and MP3 buttons won't work unless you've been to this page first (so that the relevant cookie is on your computer):
https://videodroid.org/thanks_for_support.html
(If you've got Firefox set up to always retain cookies you'll only have to go there once. If you've got Firefox configured to delete all cookies on exit you'll need to go there whenever you open Firefox and want to download MP3 files. So it's worth bookmarking that page).