To change from auto-save to showing the Save As dialog: click the Options (gear) button, and under "Show Save As dialog each time" click the Yes bubble.

Could you let me know the site where you don't get a useful file name? I'm not sure the add-on can detect the intended name, but I can take a look. (You can contact me at jscher2000@outlook.com or using the Support link on the main page.) You also could try using the IE11 button in those cases -- it doesn't change the format, but modifies the request to the server to try to get whatever format they offer to older browsers.

Hi Matchbook, does the add-on menu item appear when you right-click on an image?

If not:

One possible reason is that it is a background image behind another element; currently, the extension does not detect those. Select Open Image in New Tab first and then right-click the image in the new tab (stand-alone image viewer). Another possible issue is not be enabled to run in private windows. More info on that in the following article: https://support.mozilla.org/kb/extensions-private-browsing

Next question: do the save buttons appear overlaid on the image after you select the right-click context menu item?

If not:

You can contact me for assistance using the Email link on the main page or the Issues tab on Github (add-on Homepage link).

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/

I've added an option for this in version 0.8 (you can turn it on using a checkbox on the Options page in the Auto-close/Auto-open section. Let me know what you think.