I just think it is very seedy to bundle it with the browser and it rubs a lot of people the wrong way.
Opera bundles some features more than a browser (VPN, et al.) with the browser, similarly people don't get rubbed by it.
Mozilla puts a button which is just a scriptlet to the UI and people got tipped off.
It just sits there and can be turned off [1]. I've never experienced any aggressive marketing for Pocket by Firefox (in app or out of band). I only get a weekly articles digest which, I subscribed and, that's all.
I wish we were living in an era where free and high quality software just survive without any revenue stream and web is not a mess of standards, bombardment of adverts and analytics frameworks and other sinister stuff but, that ship has unfortunately sailed and, I personally still angry about it.
[1]: https://support.mozilla.org/en-US/kb/disable-or-re-enable-po...