What you didn't find to be true in your case - Firefox becoming slower over time (as OP says) or Firefox shipping with Pocket? Or both?
Personally my reason was Pocket incident and since then I never went back to Firefox. Checked few months ago and they still had Pocket. Besides I am pretty happy on Safari except the lack of extensions which I am kind of used to by now.
They still have Pocket, but it's both less in the way and now actually owned by the Mozilla Foundation though that's not really made obvious.
The most obvious place you'll see it is on the default new tab/blank tab page where top trending stories being saved on Pocket are listed. It's a crowd-curated list of stories that people have found interesting enough to save to read or return to later, almost like a technology news site where people vote stories up...... But no commenting.
The way Pocket is integrated in Firefox is a problem for me, may not be a problem for others and that's completely understandable.
For me Mozilla was the "open" browser (and "clean" too). Integrating Pocket in it by default, in a way that it cannot be "completely" removed, put me off. Heck, they could just ship the browser with it installed as a "normal" extension that can be "completely" removed like any other extension. And maybe disable this extension by default, or keep it enabled they'd really want it that way.
And no, if I want curated trending stories I will go the website I want to read those from. Or I'll install a relevant extension of my choice; maybe Pocket.
