So what’s wrong with making it part of Firefox? Again, the fact that most user never wanted this feature, much less as a part of Firefox. Let me explain it this way; When a user installs an add-on, they are doing so at their own choice. They understand by installing this add-on (especially in the case for third-party services) there are certain Terms of Service they are agreeing to as well as subjecting themselves to the privacy practices (and possible risks) of said add-on. If they don’t agree with these policies, they can simply choose NOT to install the add-on.
Problem is when it is integrated into the browser as Pocket has been, the user is unable choose if they want this or not. In some ways it can be seen as forcing the user to agree to Pocket’s/Read It Later’s ToS, because it is part of the Firefox browser. Yes, Pocket can and (if you never plan on using it) should be disabled, but users shouldn’t have to jump through hoops to disable something they never wanted in the first place (and should have never been included). This is a third-party feature that should have been left as an add-on.
Finally is this very enlightening blog post from Mozilla’s (or quite possibly former Mozilla) Benjamin Kerensa. One note about this post, Benjamin does acknowledge/clarify within the comments as a response to another comment that Mozilla is not getting any type of financial (though there may be other forms) kick-back from Pocket.