I've been using SmartSearch for a long time. Rather than right-clicking a highlighted string and getting only the Google choice afforded by Firefox, I could select which search provider to use. Alas, SmartSearch is a legacy (non-WebExt) add-on so it will die with Firefox 57.

A feature missing in this add-on that SmartSearch has is allowing the user to group their search URLs. That is, under the Search[es] folder, the user could define subfolders. For example, I have all my Google-based searches (general, image, news, maps, etc) grouped under a Searches/Google subfolder. All my Bing search URLs are under a Searches/Bing subfolder. Search providers for which I do not have multiple search types (differentiated by, say, images, news, videos, shopping, etc) have only a single entry so they reside under the Searches folder. This add-on will find all the search URLs under the subfolders (groups) but flattens out its context menu to show all search URLs at the same 1st-level of the context menu. I have 22 search URLs. This makes for a long list. Likely it will get longer. When I intend to search, I already know which one that I want to use so drilling through groups is easier than my eyes rolling up and down scanning a long flat list.

In flattening the list, it is generated in the order the subfolders are alphebetized as subfolders under the Searches folder. So there is still a bit of grouping. However, I would prefer the add-on support subfolders as did the SmartSearch add-on.

Thanks for making a WebExt version of a context search. I would really miss SmartSearch went it gets killed by FF 57.

UPDATE

Woohoo! Subfolders got added in the last release. I was doing some research on an upcoming vacation looking for places to get outdoors (less commercial, cheaper, and more relaxed). I was using Context Search, did some searches, and then realized that I had been selecting my search providers in the subfolders into which I had grouped them. YES! Hurrah to the author, and thanks to Zitronella on mentioning another webext add-on that this author could use as an example of code for context menu subfolders.