You're right! I found someone else with that issue and we've made little progress in diagnosing the issue:
https://www.reddit.com/r/firefox/comment...r_leaving/
tldr; appears on Xubuntu 16.04 and older Linux Lite versions as well, no other Firefox settings seem responsible, Windows is not affected.
[EDIT] FYI, I filed a bug on the Firefox bug tracker.
[EDIT] It should apparently be fixed for the next XFCE version, as this was a XFCE bug.
https://www.reddit.com/r/firefox/comment...r_leaving/
tldr; appears on Xubuntu 16.04 and older Linux Lite versions as well, no other Firefox settings seem responsible, Windows is not affected.
[EDIT] FYI, I filed a bug on the Firefox bug tracker.
[EDIT] It should apparently be fixed for the next XFCE version, as this was a XFCE bug.