When Mozilla bought Pocket, there already existed a competing open source self-hostable network service, Wallabag. Pocket is to Wallabag as Twitter is to Mastodon.
My complaint was not that the code in Firefox was proprietary, but that it integrated with (and promoted) a proprietary network service, while there was a viable open source competitor.
At the time, Mozilla's mission included the aim “to promote choice and innovation on the internet”, which this integration seemed to go against.
It literally took years to get to a point where the bundled parts were released as open source (see https://bugzilla.mozilla.org/show_bug.cgi?id=1343006 ). Obviously there are legal concerns etc, but many of the critics would have been much happier if the extension were released as open source first rather than adding proprietary code to the browser