Rated 3 out of 5 stars
In the movie showing the Add-on in operation it is a foolproof two click process.
I am finding that if I right click, and then click on add to search bar, a window opens where I can change the name, and icon, and add the keyword MANUALLY.
I managed to add a keyword manually (just do a search for say dog and replace the dog with %s) for flickr creative commons search, but some sites do not tell you the search URL, I think.
(I think that there is a bookmarklet somewhere for erm 'swapping get and post' or somesuch so that the search URL is displayed even when it is is not display by default and I used that to write keywords for maxthon in the past.)
I guess that on some sites the keyword is created automatically and on others it is not. But I have tried flickr, mozillaZine Forums, and the "search for add ons" on this site and in all cases I am being asked to provide the keyword myself.
Since I am not even seeing a "add keyword" option in the search manager this Add-on is a lot better than nothing.