I tried putting in "https://images.google.com" and all it did was send me to the default Google Image page, effectively making the whole add-on pointless. Then I put in "https://images.google.com/search?q=%s" as the address for the search engine and Firefox would not even react to me pressing Tab.

On top of that, developers should refrain from using programmer jargon when developing add-ons such as these. Extensions for web browsers should never require the user to manually type in things like "addons:extensions#search". There should be a button for when the user wants to check their list of installed search engines.