If you consider Firefox Screenshots from the perspective of a Support tool, this makes alot of sense. Customer wants to file a support ticket or bug report, needs to include a screenshot. Screenshot taken, they paste the link into the ticket, done. They never leave the browser.
Outside of the browser, fire up the screenshot tool, crop whatever you want to show, save it, upload it to imgur or website of choice, attach or paste it.
I would liken the tool to Fedora's amazing fpaste tool, which you invoke via the following:
foo | fpaste
fpaste takes the output of the `foo` command, sends it to a log file, automatically uploads it to a fedora server, and returns a minified URL that can be copied and pasted to the developer. It is an amazing tool,
provided you know what it's doing.
fpaste also takes several safety precautions such as static URLs that delete themselves after 24 hours.
So I can see Screenshots being very useful for that purpose. It just needs to be better about disclosing what it's doing.