The best method for downloading large files (including isos) is to use wget at the command line. Place the mouse pointer over the link in your browser, right mouse click, and select "Copy Link Location" (Firefox or Seamonkey). Then open a terminal session and type in:
add a space and then center mouse click to paste the link location onto that line. If the download gets interrupted wget will time out and then try again, resuming from where the previous download left off.