Works perfectly with Firefox 57.01 on Ubuntu 14.04.
Just think of doing the little manipulation before: Create the "Searches" folder in the bookmarks and assign the keywords to the search engines.
I am very happy. Thank you.

Fonctionne parfaitement avec Firefox 57.01 sur Ubuntu 14.04.
Il faut juste penser à faire la petite manipulation auparavant : Créer le dossier "Searches" dans les marque-pages et attribuer les mots-clés aux moteurs de recherche.
Merci beaucoup.

Just do its work! Very satisfied with.

Enfin. Excellent. Et merci.

Great! Without too much effort I've got back most of the functionality of the now dead – due to Quantum's limited API – add-on that used to do this. In the end I think this bookmark method is actually easier than having to add to the installed search engines, which is one aspect of Firefox that Mozilla seem to have made unnecessarily complicated in recent years.

The only thing I would suggest is rather than adding a new 'search' item to the context menu, usurp the existing one so there aren't two. Would that be possible?

Takes a bit of work, but it works well after you do some configuration. Thank you!
If I can post a couple examples for your Searches folder, the two that I use most often:

https://www.startpage.com/do/dsearch?query=%s

http://www.imdb.com/find?s=all&q=%s

The %s is the wildcard that is replaced with whatever you have selected on the page you are searching.

Many searches are available this way.

It works. Thank you^-^

A bit awkward to use, but this is not the developer's fault. In an ideal world, it would directly use the already defined search engines in the browser, but the WebExtension API does not give access to that. This add-on offers a nice alternative.

Thank you for the great work!

Questo componente aggiuntivo combina la procedura di Add to Search Bar (RIP) con la ricerca contestuale. Ogni sito con un motore di ricerca interno può essere aggiunto alla propria lista di motori posizionando il cursore all'interno del campo di ricerca e selezionando 'Aggiungi una parola chiave per questa ricerca' dal menu contestuale. Si apre una finestra di dialogo che chiede il nome x il nuovo motore, una parola chiave e la cartella di destinazione, che deve essere preventivamente creata, con il nome Searches, nel Menu Segnalibri. Da provare!

It's works! Thanks

THANK YOU, THANK YOU, THANK YOU!

ps.
possible to move this under right position in context menu? under firefox's „search google „for””? seems like a right place

Just what I wanted to make. Perfect !
I already had a folder with keyword searches. It's like you're in my head :P

Probably the best replacement for SmartSearch, but there is still room for improvement ;) For example: Is it possible to change the position of "Search for" in the context menu? I would like to have this option at the very top.

Using bookmarks for search engines is more flexible and powerful.

Can you make an option to have search features popup on the page near the highlighted word?

It'll be much quicker to search and access the addon, instead of drilling down into the context menu.

POST method aside, I like this extension because the bookmarked searches use Firefox Sync to synchronize between computers. Search engines don't synchronize and there aren't stored anymore in a folder in the profile's folder.

For the same reason, It would be great something similar for the search bar, using the bookmarked searches organized in folders.

Thanks for this. I've been concerned about QuickSearch disappearing, and this seems to solve it.
I didn't see it in the notes, but if you've already set up QuickSearch, you can simply copy and paste the bookmarks from the Quick Search folder to the Searches folder, and everything seems to work.
With QuickSearch, if you right-clicked with nothing selected, it checked for text in the paste buffer and, if found, offered to search for that text. It's pretty handy, and it would be nice if you could add that.

I like the suggestion by Teko for a "current site search" option for Google, but understand what you say about it deviating from what the add-on does. But what about this approach, which generalises the idea and might fit in a bit more naturally. Currently, the web address has %s replaced by the text selection. What if also %d (say) was replaced by the current domain name. The user could then manually edit the web addresses exactly as they like, for any search engines of their choice. For example, for a Google site search they'd change "%s" to "site:%d %s". This concept could be extended to %u (say) being replaced by the URL. It could then be used to send the whole page to Google Translate, HTML validation services etc. (In this case, the add-on would also need to work with the Context Menu you get when there's no text selection). I realise that it's a bit more work for the user to set up initially, and that there might be technical reasons why it's not suitable - but I thought I'd put the idea forwards.

Thanks for the hard work, it works very well.
just one suggestion: would you add an option/entry for searching the current site using google (Site Search)?

Works fine as described. Formerly I used https://addons.mozilla.org/de/firefox/addon/context-search-x/ which is a legacy add-on, but two of its features I wish for "Context Search WebExtension" too:
1. Favicons
2. The uppermost search engine is dedicated as standard. Therefore no need to move the mouse into the pull down menu of various search engines, but click just the main entry "Context Search" (with the magnifier icon) in the context menu and - voilá - the standard search engine is chosen and searching.
3. Of course, grouping would be a nice-to-have feature too for me.

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.