Startpage is my default search engine on all my computers and all my browsers... Just like Firefox is the first software I install on every new machine.
First because of privacy concerns.
Second because of the quality of the results.
Just try it !
Of course, having it directly in the search engines list in Firefox would ease this process !
I strongly support having Startpage as an option in Firefox's search engine selection. Yes, the Startpage extension exists, but lowering the barrier to entry for as many privacy-respecting search engines as possible is something that I believe is in keeping with Firefox's overall product model.