This would be very useful, especially on mobile versions. It would also be very useful to quickly set up a truly private browser for a friend or family member.
I believe that if Duck Duck Go has already become a standard option, it is surprising that Startpage has not. One thing that would be good on making Startpage an option is that it would be the real Startpage as there are several choices when doing an extension search and they are not all exactly the same. One of them pretty much shut down my computer. I used it because it said it was Startpage. So that would be eliminated.
Also, Startpage has a relatively decent customer care service. It is second only to its counterpart, Startmail, the privacy e-mail owned and started by Startpage. Using Startpage would bring the awareness of the existence of Startmail more onto a user's mind.
Startpage is the privacy focused search engine I use on a regular basis on most browsers, including Firefox. To search for any type of subject on a private search engine gives me both comfort and reassurance, knowing that I able to do my search on the internet with piece of mind. Startpage deserves to be included on Fitefox as the alternative search engine because it provides a great wealth of search results for the user. To search for something on the internet that is born with privacy in mind, it starts with startpage.com