With this change, it's now possible to configure the default search
engine in Firefox with
programs.firefox.profiles.<name>.search.default
and add custom engines with
programs.firefox.profiles.<name>.search.engines.
It's also recommended to enable
programs.firefox.profiles.<name>.search.force = true
since Firefox will replace the symlink for the search configuration on
every launch, but note that you'll loose any existing configuration by
enabling this.
