Firefucks
Firefucks

Firefox can't do flexbox, because why would a modern browser be able to do flexbox? Cray.
Closes #5438, Closes #5401 - same fix but vh unit because % unit is hella unreliable.

JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,JohnONolan/Ghost-Admin,acburdine/Ghost-Admin,acburdine/Ghost-Admin,TryGhost/Ghost-Admin,TryGhost/Ghost-Admin,dbalders/Ghost-Admin,dbalders/Ghost-Admin,kevinansfield/Ghost-Admin,kevinansfield/Ghost-Admin,airycanon/Ghost-Admin