Right-click on a search box > "Add a Keyword for this Search".

Edit: it then gets saved as a bookmark and you can edit it like any other.

I don't have that. Where? Algolia and google search boxes(? the input fields?) don't show anything like that.

edit: also if I modify a keyword for my opensearch/xml search engines in about:preferences#search , it won't show up as a regular bookmark. Also I can't even see the URL for those search engines.

Right click inside a search box. There should be the option "Add a Keyword for this search".

Where? What is a search box?

The search input provided by a website. You can try with the one in the footer of Hacker News and it works. See https://imgur.com/oxqMw6D

There used to be a GUI for that. Then they removed it. The functionality is still sorta, kinda available, in varying and increasingly undiscoverable ways.

Last time I checked, you had to navigate to a search engine (and/or make a search with it?) and hope its author published some magic special microformat metadata that identifies it as a search engine - then Firefox would helpfully offer you an option to add it as a search engine, somewhere in the address bar. I don't remember if it had any indicator visible by default, or if you had to right-click the address bar first.

And now I learned they "improved" this once again - hiding the feature under a right-click on a search box.

It really seems like browser vendors want to soft-kill this options. I'm just not sure why, especially when it comes to Firefox.

There's a firefox plugin [1] that lets you add custom search engines; they will be visible in the settings menu under "Search Shortcuts" where you can set the keyword to trigger the custom search.

[1] https://addons.mozilla.org/en-US/firefox/addon/add-custom-se...