I gave this 1 star initially because I couldn't figure out how it worked. Turns out that theres instructions which I found on the downloaders website:

"The RedGifs Downloader extension adds a convenient download button to RedGifs pages, allowing you to easily save videos and images directly to your device. It also shows your download count and total download size."

So yeah, it adds a button to the actual redgifs website on the right by the other buttons like fullscreen, mute, like, etc.

Changed rating to 4 stars. It would get 5 stars if the author actually put that instruction in the description of this extension.