Proprietary? By that standard, Firefox is proprietaty because they roll their own browser. Just because it's custom doesn't make it proprietary.
And I also got burned by the very badly managed WebExtensions thing. I can totally see where GP is coming from.
As for Firefox including Pocket, they own Pocket for starters, and it's less annoying than Edge or Chrome pushing their analytics filled services. There's a lot less cruft on Firefox than in other browsers. If I really wanted I could use one of the GNU forks of Firefox, though I'm not sure how much more privacy conscious they may be aside from a bunch of brand rework.