Thank you for your feedback. For animated webP, the "IE11" button works the best for saving in an animated GIF format, assuming the server offers one, but isn't perfect. For WebM, I could try linking to an online conversion service but I'm not aware of one.

The Save As dialog takes advantage of Firefox's built-in behavior to remember your last used folder per website. I actually don't know whether I can override that in the add-on; I'll need to look into it.