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.
Starting today (April 21) we're running a 30-day experiment with Startpage — if you're in Germany, France, the Netherlands, Austria, or Switzerland, you may see it as an option in the address bar via the unified search button.
Check out the following discussion thread to learn more and to share feedback directly with the team working on the feature: Try Startpage Search in Firefox 150!