"Firefox can't find the file at gogdownloader://faster_than_light/p272."
This is a problem with Firefox having incorrect entries for the gogdownloader:// protocol. Go to Options->Applications, find gogdownloader and change action to the default one, should work. If that fails, remove the entry in firefox and reinstall the downloader.
My Downloader is Chrashing at startup no matter what I do!
Could it be that you have it set to download to an external drive, that is not available atm? If so, reconnect the drive and try again.
Alternative tests, try starting the downloader without adding a game (use the start menu), and/or verify your .NET installation. I assume that the downloader was working before but it started crashing recently, do tell if you never got it to work.
Also, when reporting problems, it helps if you at least mention the OS you are using.