Did you remove the IMDB cookies too?
those might be what is keeping that setting in memory.

did you try to go there with IE to see if it does the same thing?

I registered there to check it & it still is normal for me.

Did this change just happen?

I do think it is something with your FF settings or the cookies on your computer. It the remove cookies don't make a difference...
Only other thing might be to download another browser like Flock or Opera {flock is most like FF}

I didn't see any settings on the IMDB site for test/display options..
So I'm at a loss as to what else to suggest.