If the option doesn't pop up, save file as it is then upload it here and convert to .gif format and download, super fast and easy, it's what I do with all the mp4s and webms i want as gifs. Think you get 10 file converts a day, 25 if you get a free account. cloudconvert.com/webm-to-gif
I thought that might work as well, but for some reason trying to change webms and mp4s to gifs via this way just corrupts them when you try to access them, as shown in my pic