An epiphany: An Epiphany regarding Purebrowser – Purism
To make our CSO’s vision a reality we’re planning on moving away from Firefox ESR as our default and towards the default GNOME Web browser called “Epiphany”.
Purebrowser removed: Purebrowser removed after update
I don’t know whether the change is reflected in Amber yet.
That doesn’t mean that you can’t use Firefox ESR or, alternatively, a more current version of Firefox. The essence of freedom is your right to use whatever browser you like!
However I would guess that Purism effort regarding a default, out-of-the-box, secure, private browser with PureOS will be devoted to Epiphany (Gnome Web) - and that they won’t be spending effort on what add-ons are or are not applied to Firefox.
Hence, if you want to query Libre-JS v. uBlock Origin … you would need to see what they are supplying with Epiphany.