Firefox – Files downloaded with Firefox are often corrupt

For the past six months or more, when I download a file with Firefox, it will usually be corrupt. During those months, I've always kept Firefox up-to-date. I'm currently running 3.6.6 32-bit. Here are some details:

1. If a download is corrupt and I immediately start the download again, and the source URL and destination path are identical, then the download finishes instantly and the file is then valid.
2. It only happens for Firefox. I have no issues with IE, with any HTTP download managers, or downloads using any other transport protocol.
3. It happens on more than one system, each system is completely different (one is hardware and one is a VM) and I run a totally different software firewall on each. They are both running Windows 7 64-bit, however.
4. It happens even if I run Firefox in safe mode (no extensions or plugins).
5. Regular web usage w/ Firefox seems to exhibit no problems. Web pages and images do not appear corrupt. However, downloads of extension updates sometimes fail their integrity checks.
6. For what it's worth, anti-virus and malware scans come up clean and I take fairly strong measures to protect my browser and my system.
7. It happens for both HTTP and HTTPS downloads.

I ran some tests downloading the same 2.5 MB .zip file 6 times in a row (renaming the source file each time so it wouldn't cause #1 above to trigger) and compared the corrupt files with a valid file in WinMerge. Each time, the corruption is different. Most of the file contents match, with segments always missing from the corrupt file. In one of the six cases, however, a segment of file that shouldn't exist was inserted into the corrupt file, as well. The chunks of corruption, whether missing, erroneous, or added, appear in WinMerge to be similarly sized, perhaps related to Firefox's download block size?

While a number of things could explain some of the symptoms, I cannot determine a cause that would fit them all, particularly #3. Any help would be greatly appreciated.