Version 1.0 adds a "Save as IE11" button which helps with some animated GIF files (such as on Tumblr). Sometimes you need to cancel the first save dialog and click it again to bypass Firefox's cache and get the original .gif file.

Alternately, as mentioned in the description for this extension, you can try my other extension, which discourages sites from sending you WebP images when the original is a GIF: https://addons.mozilla.org/firefox/addon/dont-accept-webp/