If I were Mozilla I would support Bing, Yahoo, Google, Duckduckgo, Yelp, Ecosia, Qwant, Startpage, Yandex and Wikipedia as search engines out of the box. I would also pursue revenue sharing agreements with each of these search engines to make up for the fact that Google will no longer be the biggest donor to Mozilla in a years time.
Yes, please. I rarely, if ever, use Firefox on my Android phone because it doesn't mirror the Start Page I have set up on my desktop/PC version. I've tried the "Desktop Site" button in Settings, doesn't do anything.
Having said that, my browser opening Start Page on PC has been the MyYahoo consolidator homepage for a very long time, but Yahoo ended that a couple weeks ago, which totally sucks, and now there's quite a bit of feedback on Reddit and elsewhere about the loss of MyYahoo after decades. Hopefully Mozilla would take a look at the now-gone MyYahoo landing page functionality, recognize all the refugees it created out in internet land, and see an opportunity.
You can manually add the search engine. Open the search engine you want, click on the address bar, and then press the '+' button at the bottom of it to add the search engine.
Startpage values your privacy but it isn't a great search engine as it just combines results it gets from two horrible search engines, Google and the microsoft default. But really, we should have flexibility to set our search engine and not just be limited to the few not very good search engines we are presently limited to. Mojeek would be my choice to set as my default engine -- it has privacy protection and it actually is a good search engine.