Mozilla's add-on page says this extension was last updated on July 16, 2015. Wrong. Go to the author's web site (http://twofoos.org/content/smartsearch/) which says the extension was last updated on July, 26, 2014. So the author has not updated this extension for 3 years. If there were no bugs then that would be okay; however, it is a "legacy" extension which means it is NOT a WebExtension coded extension. That means this extension gets marked as a "Legacy" extension as of Firefox 55. It also means this extension will *die* when Firefox 57 no longer allows non-WebExt extensions.
I liked this extension because it afforded me the right-click search that Internet Explorer had and which Firefox lacks. I could even group search URLs into folders under the context menu entry, like all Google, all Bing, and all <someothersource> grouped together. This made my 22 search URLs feasible; else, I'd be stuck with a huge list of search URLs and have to waste time with my eyes rolling up and down the long list. Since this is an abandoned extension and will never be a WebExt extension, I'm forced to start looking elsewhere for a WebExt solution for context menu based searching. I can use the keyword in a bookmark for the search URL but that has the nuisance that I have to first copy the text from the web page, enter the keyword (as a prefix) in the address bar of Firefox, and then paste in the search string text. So I don't lose the use of the keywords for my search bookmarks. I lose the ease of right-clicking and using a list from the context menu.
5 stars for being a great extension. Minus 1 star for abandonment. Minus 1 star for not being a WebExtension add-on (and no development to create one).