A Wall of Fire is not hindering terrain because the power does not specify it as such. So, no saving throw.

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This is an interesting arguement since Merriam-Webster says this about Hinder (yes, I got the real definition).
transitive verb 1
: to make slow or difficult the progress of
: hamper
2
: to hold back
: check
intransitive verb : to delay, impede, or prevent action
Now, technically, knocking someone off a cliff isn't slowing their progress...in fact it is making them progress very quickly downward.
So I suppose it depends on what you are hindering the target from. If it is strictly movement, then only forced movement into difficult (movement reducing) terrain (put in the game, you don't save vs. being pushed into difficult terrain!) or into a pit 1-square wide pit (being that the only movement in the pit would be to climb out, and which is slower movement)is hindering them, so off cliffs would be just fine.
If you mean hindering their victory, I would think forcing them into damaging terrain counts.
But the, I guess characters who can teleport aren't hindered by difficult terrain, so then you could move them into rough terrain....
Perhaps they should have used a "damaging terrain" instead of "hindering terrain" since that seems to be what they meant: squares that have damaging or life-threatening qualities if you enter them.