Hi trhv,
For what it’s worth, you should be able to use StartPage with PureBrowser, without need for the AddOn (you may have already found this link by now):
The thread includes instructions for setting security.ssl.enable_alpn to false, which I think, from reading closely, might not be entirely necessary to get StartPage set as the default search engine.
I have to admit I don’t have my Librem laptop yet, and need to give PureOS a try on another machine until the Librem arrives.
I hope the above link is of use to you.
- Clark