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.