The autohide fonction doesn't work when you open a new window and/or start Firefox.
You need to open then close the setting menu to make the hide automatically works again.
- Same issue on each Firefox version (Stable, ESR, Aurora, Nightly).
- Same issue on Debian Jessie and Fedora (19 > 22).
- Same issue on a shared profile on a Windows 8.1 and 10/Fedora dual Boot: It appears on both OSes if the Firefox profile is shared.
The autohide fonction doesn't work when you open a new window and/or start Firefox.
You need to open then close the setting menu to make the hide automatically works again.