Saving a video off imgur works about half the time in firefox, based on ???. There's a bug for it but it hasn't gotten much attention. It's a problem of imgur playing games with headers, embedding a video inside a web page and using the exact same URL for each. I haven't had any issues in chrome.

Twitter's just putting dumb empty divs over the content. And they make it impossible to get the source gif even in cases where the video version is larger and lower quality. Thanks twitter.