No matter how many times it's disproven, this meme just never ends, does it?
The Pocket client code is free and open source, just like Firefox itself[0]. If the user uses the Pocket feature, that code makes HTTP requests to another server, which runs a mixture of FOSS and proprietary code. If the user does not use the Pocket feature, it does not.
Remember that we are talking about a web browser, the primary function of which is to do exactly that: make HTTP requests to another server that may be running proprietary code[1].
[0] Pocket's client is arguably more free than Firefox is, if we look at the fact that Iceweasel is distributed with the Pocket logo included whereas Debian cannot include the Firefox logo for its patched builds.
[1] Unless you only use your browser for accessing http://fsf.org, but at that point you're probably using Lynx or Seamonkey anyway.