You can change between ones that you have a search tools extension for, https://addons.mozilla.org/en-US/firefox/extensions/category..., but you can't just put your own custom search engine in.

That said, keyword searches are awesome and I use them for a whole lot of things, including effectively custom search engines. I have keyword searches against, e.g., internal confluence docs so I only have to enter "con <thing I'm searching for>" and firefox will load the confluence search page, or "people <name>" to bring up our internal phonebook with a search for the employee. I don't think I've ever needed to add my own entirely custom search engine to the browser.

I have UI to enter a custom URL for search in Firefox for Android, with no extensions beyond Privacy Badger, Unhook, and uBlock Origin- interesting.

Double checked mine, same. How bizarre. Desktop doesn't have it by default but mobile does.