Like so many other Firefox extensions these days this one also tries to corner you into paying a fee for continued use. After you hover over a certain number of clips a pop up will tell you you have hit your limit and need to subscribe to continue watching them.

This paid requirement is not mentioned anywhere on the addons.mozilla page for this extension. You do see a gold subscribe button in a couple screenshots but that's it. This is a shady extension and there are better ones out there that don't force you into a paid subscription to use.