Opensearch engines could be added to firefox from this page: https://addons.mozilla.org/en-US/firefox/search-tools/ .This in practice was just an extension but a different schema. They probably just want to consolidate and maintain only one schema for all extensions.
Now, it is still truly surprising to me that it's easier to use third-party search in Chrome than in Firefox, given all of Mozilla's talk about the Google monopoly. In Chrome, adding a search engine is as simple as going to chrome://settings/searchEngines and adding a string like "bing.com?q=%s" while in Firefox the only way I can find out so far is packaging an extension and loading it up.