Do you remember if it was possible to edit or add custom search engines from GUI before? I remember having them, but I can't find it. Also it seems to me as a basic feature and not a "killer feature".
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.
[1] https://addons.mozilla.org/en-US/firefox/addon/add-custom-se...
Edit: it then gets saved as a bookmark and you can edit it like any other.
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.