I investigated the bug and found the cause:
hr with width 8388599 or less is OK
hr with width 8388600 or greater cause problem
Body background color must be set!
It is probably a 32 bit bug because 16777216/2=8388608.
(bit of sign)
And perhaps hr has padding or margin (2*4=8).
Someone could try it with a 64 bit version of Firefox..
I investigated the bug and found the cause:
hr with width 8388599 or less is OK
hr with width 8388600 or greater cause problem
Body background color must be set!
It is probably a 32 bit bug because 16777216/2=8388608.
(bit of sign)
And perhaps hr has padding or margin (2*4=8).
Someone could try it with a 64 bit version of Firefox..