Did you try to reload the YouTube page via F5 or Ctrl+F5 (bypass the cache)?

YouTube makes changes to their servers all the time and that can prevent download extensions from working properly.
Extensions need to be updated to fix issues caused by this, so make sure to check if there is an update.
Otherwise you will have to wait until the extension developer has released an update to fix this or check for a possible beta version.

I tried reloading the page, but I didn't even know about F5, so I'll try it. I think I do have the latest version; I checked, and have it set to do automatic updates.

Anyway Thank YOU! I'll go try it and let you know. Thanks again,

Oh man, I'm in big trouble. Something fried mycomputer--could it have been downloading viders or something like that? I'm extremely careful downloading apps & such unless I can get it right from you. Meanwhile, Firefox seems to have forgotten me. My configurations, along with all my bookmarks and history and items on the bookmarks toolbar are gone. I mean, I'm sure they must be there somewhere, but can I get back to being me? I assume this has something to do with this laptop having suddenly forgotten my passwords -- to the computer itself, so Firefox and everyone else is seeing me as someone else.

Can you please help me reclaim myself? Thanks!