One of Mozilla's major value propositions is a privacy and security focus. Lots of people are interested in Firefox because it
doesn't stream tons of data about browsing sessions upstream. It should always be obvious when a user is uploading information to the internet; the intuition on snipping/screenshot tools is that they perform actions locally.
I take screenshots of internal graphs, admin pages that may contain pseudo-private endpoints and admin account information all the time ("Logged in as $ADMIN_USER", account numbers with vendors, etc.). Sharing those in a private chat or email is a big part of my screenshotting use case. People screenshot a lot of private data.
I don't think Mozilla is wise in making "send your image to a public bucket protected only by the obscurity of its URL" the default behavior, at least not without making it obvious that this is what's happening. This sneaky exfiltration of private data is a large part of what people hate about Google and Facebook.
Maybe there could be an opt-out on first use? I get that your average grandma isn't going to understand the distinction and will want something she can share (but she's also probably just trying to get to email or Facebook anyway), but this is a real potential security problem for your users.