The problem here is that Safari tries to use the page title as the filename. The bug in this particular case is that it doesn't seem to remember the 255 character filename limit that is inherent in an HFS+ filesystem. Simply inputting a different file name in the save dialog fixes things.

I wasn't able to replicate the thing you were talking about with backspacing and re-entering the last character, but I do know that it would refuse to let me save, but then if I deleted enough characters from the proposed filename, it would eventually agree to save. When I checked, the magic length seems to be 255 characters.

Yep, that's it. Firefox uses the end part of the URL for the default file name, whereas Safari uses the title. It's the length of the file name that causes the save button to not be active. Shortening it fixes the problem. I'm not sure it's a bug necessarily, but I suppose Safari should replicate the Firefox behavior or automatically shorten the name to 255 characters.