The fact it ought to be an extension that pocket users could add by visiting addons.mozilla.org but instead it was rolled into the core browser.

People assume Pocket offered Mozilla a lot of money, as there's really no other reason Pocket shouldn't be an extension.

People are naturally nervous when browsers start deploying third-party code in exchange for cash. Especially when auto-update mechanisms are used.

Some would see that as an important line to cross - will they be able to resist the much larger sums they'll be offered to "help users see more relevant ads" and "help users keep safe from viruses"?