First, I didn't realize the button bar was scrunched in a way that makes it unusable on those images. I'll have to investigate.

Second, by default, if you Shift+click the menu item, it bypasses the button bar and saves as PNG. You can change which action occurs on a plain click versus Shift+click in the options. (Another reviewer shared an example of where to change this: addons.mozilla.org/en-US/firefox/addon/save-webp-as-png-or-jpeg/reviews/2120797/ )

I have a couple pending requests to have sub-menus as an alternative to a button bar, since not everyone finds that design attractive, but I haven't found time to work on that.