Zero. Because, once again, the open source Pocket code contained within Firefox is only ever activated if you specifically choose to use it. It's not even loaded due to lazy loading.
> How many of those users were forced by updating Firefox?
"forced" would be zero
I bet the number of users would be much smaller if there was no such button.
How hard is that to understand?
> Force [verb]: make (someone) do something against their will.