The problematic feature is the "Save" button that appears when Firefox users take a screenshot.
The issue is that the Save button doesn't save the screenshot to the PC, as most users would naturally expect, but uploads the image to a Mozilla server.
This is both a privacy violation, as some users don't appreciate being tricked into uploading sensitive images saved on remote servers, but also an incovenience as users would still have to download the image locally, but in multiple steps afterward.
Users have complained about the button's label for months, and for good reasons. An attempt to replace "Save" with "Upload" was made this last September, but the update never made it into Firefox's Screenshot tool.
However, news has emerged this week that the "Save" (actually Upload) button's days are numbered. According to notes from a Mozilla meeting that took place this week, the backend servers of the Screenshot utility are being shut down.
The servers being shut down is a matter related to Mozilla dropping the Test Pilot program, where the Screenshot tool was also initially developed since in 2016, before being rolled out to all users between Firefox 56 (September 2017) and Firefox 59 (March 2018).
With the Test Pilot being wound down, the server shutdown is scheduled for the second half of 2019, however, the Screenshot utility will remain in Firefox and continue to work, albeit without the server upload capability.
This means the blue Save button is also going away, and will most likely be replaced with a "Download" button, like the one below.
This is the same button that currently appears if users disable the server upload functionality via the about:config setting of extensions.screenshots.upload-disabled.