This is problem with Firefox, not with FDM so you should better ask on some Mozilla forum.
What Firefox version are you using? Do you use any tab control add-on like TabMixPlus?

Guest wrote:I would like to see an internal 'known servers management'.

You have already "Site Manager" tab in FDM.

Options | Settings | Downloads | Essentials -> Prevent computer from stand by while downloading

I downloaded hundreds of files greater than 100 MB with no problem at all, so it is definitely problem on your side. You must ask someone to check your system and network in place. Sorry, I cannot help you any more.

I removed all the posts with quoting tests.

Could you provide any link to check?

Could you provide any link to check?

Switch on silent monitoring.

Click the link you selected to download and when you see IE dialog window to download file, click "Open" button instead of "Save".
And always update your Windows and IE.

I think you might have stupid antivirus.

And what about system restore?

You should properly configure your firewall/antivirus/internet protection software.

Please consider adding an option to disable sparse files for torrents. Sparse files, being downloaded randomly, quickly become heavily fragmented, which is rather big cost for disc space. Please consider buying a new disk or providing a good link for your assumptions. Here are some opinions based o...

FDM is OK. I downloaded the file from your link in 4 sections with speed about 128 KB/s for single section, that is 512 KB/s total (512 Kilobyte/s gives about 5 Megabit/s bandwidth). I have 10 Mbps connection, so 5 Mbps is probably maximum you can have from that site. Now when outpost and kaspersky ...

Did you try to search forum?
search.php?keywords=portable&terms=all&submit=Search

And did you look at the FDM menu?
File | Create portable version