I also would love to see Startpage, among other privacy-focused search engines, introduced as default search engines. For a browser that values privacy, it's odd that Startpage isn't already an option.
Custom Search Engines: Firefox now supports adding even more search engines. To add a custom engine, right-click a search field of a supported website and select “Add Search Engine”, or go to Settings > Search > Add (below the search shortcuts table) to manually enter a search URL.