What do I do when I see something like this? It really sucks if you want to download something and this one shows up.

What do I do when I see something like this? It really sucks if you want to download something and this one shows up.

The most likely explanation is that the server you are downloading from isn't telling Firefox (or any other browser) the size of the file, so it cannot compute the estimated time.
The only action you can take is to complain to the administrator of the website. You may tell their support that they are missing the Content-Length header when sending you the file.