I know this website isn't about motor racing, but I am reminded that the great world champion Graham Hill, father of Damon, was once interviewed during a race meeting in the USA, and was earnestly asked by the reporter from the local paper what he felt was the significance of the back straight of that race track. To which he replied 'well the significance of the back straight is that it connects turn five to turn six.'
We are told that his moustache bristled as he said it, but I can't see how anybody could tell that. I think they just meant that it should have done.
Anyway, my point is that whether a top tube slopes depends on the height off the ground of its front end relative to that of its rear end. And for any given size, the height of the front is governed by the length of the head tube (fork lengths being pretty standard back then), while the height of the rear is governed by the amount of seat tube extension above the top tube (bb drops being similarly standardised).
As Joe Murray said, designs derived from road bikes had very short head tubes combined with almost no seat tube extension. If designers then decided that for mountain bikes the head tube should be longer, and there should be a greater seat tube extension above the top tube, then the result as Graham Hill would tell you, were he still with us, would be a sloping top tube. Which was the cause and which was the effect I couldn't tell you.
So this is my many-worded way
of suggesting that we shouldn't look for whether the top tube slopes or not, but rather we should judge from how long are the head tube and the seat tube extension.
It would be nice to see pictures of that Fire Mountain survivor, and to have its measurements.