As you dload a file the kbps display starts at 1, increments by 1 until done, acting as a counter. We should be able to get a rational value after a couple of seconds.
Was this observed on a small xpi? Anyways, the new download UI will land in mozilla0.9.1 and may alleviate this problem. We should re-evalaute this problem after the new download UI has landed.
Reporter: Does the problem exists in newer (1.5) builds? If it works, then please close the bug as WorksForMe else, let us know a bit more.
As the reporter doesn't seem to care I mark this WFM Reopen if your opinion is different and you can reproduce this with a current build.