about:config was made for this sort of thing.
I would think...
screenshot limit was reached after image height of 10000 was reached. I'm assuming the height is what killed it. (I hat dpr set to 3 or so)
Maybe it already is a setting there and i just don't know the name.
I made a posting about it over at discourse.mozilla.org and was told this may be the right place ...
this or bugzilla... and I'm just not sure this would be considered a bug.