The fix the guy below me posted totally worked.

Firefox has F'd some addons. Instead of my previous little rant...

Here is how to fix this if it doesn't work anymore for you:

1. Type about:config in the URL bar.
2. Search for xpinstall.signatures.required
3. Change the setting to false

Enjoy!

Edit: thank you, second guy from above me!

Works in YouTube.

good

Cannot download

Exelente inmejoranle

Brilliant, installed and works right away.

Merci !

Youtube работает с этим расширением. Круто!

youtube可以后台播放了，很好用

Nsns