Firefox's screenshot tool has a lot going for it, but after two days of trying to use it I gave up and went back to using Ksnapshot (now Spectacle) for the near-constant screenshotting I do, all day long: that's because when you hit "save" in Firefox's screenshot UI, it didn't save it to your hard-drive, rather, it uploaded it to a Mozilla server, which, in addition to being time-consuming and stupid, was also a potential huge privacy risk (if, for example, you were screenshotting a sensitive document to retain for later).
Thankfully, this will be fixed, after months of user complaints, as part of the shut-down of the Test Pilot program, which runs the servers that the screenshots were uploaded to.
On Zdnet, Catalin Cimpanu calls this a "dark pattern," and it's easy to understand why: so many online services try to trick you into using the cloud, storing data remotely even when there's no good reason for it, to train us to use other peoples' computers rather than our own.
I don't know that Mozilla has that same motivation, but this really was a terrible piece of UI with real risks to users, and it's so good to see it finally dying in a fire.
You can turn off the antifeature right now by going to about:config and ticking on the extensions.screenshots.upload-disabled setting.
Firefox to remove misleading button after months of complaints [Catalin Cimpanu/Zdnet]