Still needed, the official Startpage search continues to use GET.

Exactly what I was looking for! Awesome dev! I just created a mozilla account to write this: PLEASE do NOT delete or abandon this addon as long as Startpage does not support this incredible important privacy feature in their offical addon! Keep up the great work and thank you for investing your time in this!
PS: Maybe (but only maybe) you could implement options like other languages, regions etc. in the POST request