Launching firefox always causes it to create the "$HOME/Desktop" folder (even when XDG_DESKTOP_DIR is set to something else). Setting the system to a locale that translates "Desktop" to another string might also cause this, but I have not tested it.
This still happens even on fedora 25.
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This still happens on fedora 27.
This message is a reminder that Fedora 27 is nearing its end of life. On 2018-Nov-30 Fedora will stop maintaining and issuing updates for Fedora 27. It is Fedora's policy to close all bug reports from releases that are no longer maintained. At that time this bug will be closed as EOL if it remains open with a Fedora 'version' of '27'. Package Maintainer: If you wish for this bug to remain open because you plan to fix it in a currently maintained version, simply change the 'version' to a later Fedora version. Thank you for reporting this issue and we are sorry that we were not able to fix it before Fedora 27 is end of life. If you would still like to see this bug fixed and are able to reproduce it against a later version of Fedora, you are encouraged change the 'version' to a later Fedora version prior this bug is closed as described in the policy above. Although we aim to fix as many bugs as possible during every release's lifetime, sometimes those efforts are overtaken by events. Often a more recent Fedora release includes newer upstream software that fixes bugs or makes them obsolete.
Still present in current firefox on fedora (see also still open upstream bug).
I cannot reproduce this issue any more. Please tell me if you can.
It looks like it has been fixed in the meantime. Thanks for checking!