[lpln3452]

Firefox still lets you do this.

You can add any URL as a custom search engine by providing a string template for the query.

It doesn't have to be a formal "search provider". Any URL that accepts a query string will work.

[a022311]

IMO the problem with Firefox is that custom search engines in Firefox can't use POST requests, even though it's supported. You may want to check Mycroft Project [1] out for that.

[1]: https://mycroftproject.com/