I've encountered this problem too. Either the video stalls out at 50%, in which case the answer is to shut the browser and re-start the download (it doesn't add to the 3-day total, if that's a concern); or, it stalls at like 80-90%, in which case just press the re-start button (not the refresh button) to re-activate the download (it picks up where it left off and everything is good).
EDIT: Removed secondary concern. It seems as if recent updates to Firefox could not recognize the full files of the SLR conversions. No problem with the original file, only with the conversions ... I have no idea what causes this but the solution is Free Download Manager (FDM). It is easy to learn and reliable and fast. However, it requires an Add-on for Firefox, so don't forget to install that.