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

Excelente complemento de lo mejos