I suspect things have changed since I posted this. I actually stopped using PureBrowser, so I cannot try or look at anything.
If you have not tried it yet, @mladen later posted a work-around that might make adding extensions easier. It is at the bottom of his “Firefox Compat Mode” fix page here. Maybe it will help.
Also, a handful of the more popular privacy-related add-ons are in ‘green,’ e.g. uBlock Origin, uMatrix, HTTPS Everywhere, and Privacy Badger. Be careful, however. I found, at the time, those interfered with my Quantum installation on PureOS. (I did not try hard to debug this; I eventually just uninstalled the webexts and PureBrowser.)