Broken / stalled downloads don't sound like a Firefox problem to me. If you can reliably get a DL to fail in Firefox (that works fine in another browser) and reproduce it, file a bug :-)
I can't get things to reliably fail, admittedly the connections I am on are flaky. I have been traveling around the world for the last 8 months. From direct experience I know that Chrome handles flakey connections much better than FF. I routinely get downloads that are completely stuck in FF, showing 20KB/S as a download speed but the file hasn't been touched in 5 minutes and nothing gets written to disk and the activity monitor shows 0KB/s of network traffic. I would open a bug, but I would have to write a server that would expose the problem, no decent repro case and it wouldn't get fixed.
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edit, Many times I have to pause a download and then resume to get the dl unstuck.
