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.

Great job. I was looking for this since no other addon is a webextension. Just one question though, but let's take it to the support page:

https://github.com/NumeriusNegidius/Context-Search/issues/1