I am running Linux Mint, and I did not expect adding a custom search engine to be missing from Firefox. There are plug-ins, but I don't like adding plugins.
Open a search, right-click on the address bar. The author added the magic metadata needed, so Firefox should be able to pick it up and offer "add search engine" option in the context menu.
I know. It's an idiotic UX regression over being able to configure search engines yourself in the Settings screen. Takes control away from the user.
I did not expect this to not be available.