Was alarmed to learn that Mycroft Project search bar options no longer works, while recreating my Firefox profile. Apparently Mozilla is depreciating Opensearch XML addon support? That's dumb.
https://web.archive.org/web/20200629185533/https://discourse.mozilla.org/t/deprecation-of-opensearch-xml-add-ons-current-users/46868
Found this extension and it does the job of creating custom search bar options perfectly. Highly recommended!

A great tool that perfectly does its job

It's a shame Firefox no longer lets me add DuckDuckGo UK (I used to be able to search on here and add it just fine), but this extension helped me. Mozilla Firefox has really dropped the ball here. Well done Tom Schuster for enabling a customisable work around.

Doesn't work on Firefox 78+ and had to use "Add Search Engine from Mycroft Project" instead.

Edit: Works now, thanks for prompt fix.

tks, working perfect for searching product on marketplace
ex: tokopedia.com, bukalapak.com, shopee.co.id, lazada.co.id

is there a way to synchronize custom search engines between different devices?

Think back to the days before rolling releases and remember the simpler times. Times when you could add any site to your search bar in a single click. Those were good times. This gives you a dose of that sweet nostalgia of having a powerful URL bar, but with the added fun of being able to customize the handling a wee bit more by yourself. I hope some day we'll even get to a point where we can set keywords ourselves.

great for switzerland > so i put google.CH and delete the .com.
now it works like i want
why FF dont have this in the options ?

Works very well. You can uninstall and the search engines will stay.

must be default in firefox!! thanks, really only the way to add missed search engines! <3
Here is my answer:
@How to add a custom search engine to Firefox?@

https://superuser.com/questions/7327/how-to-add-a-custom-search-engine-to-firefox/1474187#1474187

and here as well:
@Firefox: add Google search engine - why no such option in search/address bar?@
https://superuser.com/a/1549586/230571

添加时出现这样的提示：SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

It does what it says, and requests no unnecessary permissions. Excellent add-on!

Works like a charm.

works wonderfully, though some UI decisions (like changing %s to {placeholder} when switching get to post) are questionable