If you're managing your Firefox profile with Home-Manager and you want to set Kagi as your default search engine, here's a snippet:
programs.firefox.profiles.home-manager = { search = { engines = { "kagi" = { urls = [{ template = "https://kagi.com/search?q={searchTerms}"; }]; definedAliases = [ "@k" ]; iconUpdateURL = "https://kagi.com/favicon.ico"; updateInterval = 24 * 60 * 60 * 1000; # every day }; }; default = "kagi"; privateDefault = "kagi"; }; };