The download managers don't require
the server to support resuming
That's not correct. If a server doesn't support resume then download managers cannot resume downloads.
When is Firefox going to finally add
the ability to resume downloads
Firefox does resume downloads. The problem is that many servers refuse to resume downloads even if there's a break in connection - the time period differs from server to server.
( TED's servers immediately comes to my mind, if there's break in connection for about 5 seconds, the download is ruined and I have to restart the download from start.)
I just tried Free Download Manager
(FDM), again, having the Firefox addon
Flashgot use it. The download gets
passed to FDM, and fails, giving the
error message "access denied, invalid
username or password.
I also tried DTA instead of FMD with
Flashgot, and I get an "access denied"
message in DTA.
Few servers inspect the HTTP headers for the referrers, and block if the referrer is not a browser. I guess while transferring the download, the header gets mangled and the referrer becomes the download manager, instead of the browser.
The failure message in the Firefox
Downloads window is source file at
http... could not be read.
That's become the download links are autogenerated, are valid particular combination of parameters ( perhaps some session id ? I'm not sure), and are not persistent. Once the control of the download has been handed over to the browser, the links expire, and hence you get the error message.
My suggestion: Try IDM. It's one of the better download managers that I've used. If the problem still persists, my suggestion is to use an alternative link to download, or maybe switch to a wired connection while downloading.
Best Answer
Youtube sometimes changes its protocols, which can make video-download tools stop working until they are updated. FDM might just be in such a state. If this is the case, you can either wait until it is updated, or try another free tool, such as DownloadHelper, which seems to be working fine now.