I agree with Bob that it's normal MO with Firefox.
The setting mentioned and discussed at Mozilla disables or enables the on-demand scan at the end of the download; i.e. Fx invoking an AV scan. As mentioned in a KB article, one of the links at the support thread, and as I alluded to, there could be some delay before the file is saved, caused by the scan, or, apparently, in some cases the file is not saved properly. Direct link to the KB: this section
One should always have real-time AV software, I agree with you about that; in the best worlds, this will give protection when one is trying to access a file; and, as you know, it is also possible to do an on-demand scan of the file after download.
I agree that it's perhaps not the best solution (to disable the Fx setting, but then some argue it's just for show), but as long as there is real-time protection, and with possibility to do on-demand scans, there shouldn't be any concern. After all, the AV software doing the initial scan, invoked by Fx, is the same one doing a manual on-demand scan. If it doesn't find anything in the first case it won't find anything in the second, if everything works as expected. The difference is that with the first scan, at the end of the download, it seems to get in the way of Fx saving the file.
It's possible that some AV software has problems with this, or with some configurations (CPU speed, RAM etc.). Could perhaps also be something with permissions for the folder, or being downloaded to a USB memory, but it seems like you have tried different folders, and also have had success with smaller files.
It's not a giant leap to also assume that larger files takes a bit longer to scan, thus there could be quite some delay, causing problems as mentioned initially. You could do some tests, and if it works with the Fx setting disabled, it points to the AV software being the culprit (given the config).
It's a long time since I used IE, so I don't know if it also triggers an AV scan; if it doesn't it may explain why it works without problems using IE. And then, if IE doesn't launch the AV, it wouldn't matter much if it's disabled in Fx.
(In the support thread, mentioned above, Jefferson also linked to a general article about download problems, and I guess you have looked at that one as well.)
Byelingual When you speak two languages but start losing vocabulary in both of them.